Modifying BMP color palette and saving as correct format

Question

An old abandonware game uses an image's color palette as the game's color palette https://ufile.io/su465 <- Here is said file

Windows 10 does not recognize it as having a file type, but using online file-type checkers suggests its a Windows 3.x BMP file having a DIB file header. I would appreciate assistance in figuring out how to go about modifying this file's color palette and being able to save it in the (same) correct format, so that it is still readable by the game.

Initial experiments show that it can be opened with an editor such as GIMP, the palette changed, and saved as a .bmp image, but I cannot find any settings which produce a .bmp of the same format as the original and thus it is read incorrectly.

Answer 1

It's a 23x23 bitmap using 1-byte per pixel.

The first 14 bytes of the file are the BITMAP header itself. (yellow)

The subsequent 40 bytes are the DIB header representing a serialized BITMAPINFOHEADER struct that identifies the images as a 23x23 image using no compression and 8 bit per pixel. (red underline)

The next 1024 bytes is the color palette. 256 entries. Each entry takes 4 bytes. That is an RGB (or is it BGR?) followed by zero byte. (Blue)

Then the image bytes, which is 23 rows of 24 bytes each. Rows have to align on 4 byte boundaries. (Black)

So if you want to replace the image palette, fill in the 1024 bytes starting at offset 0x36 with your own pallete data.

Answer 2

You can load the image and change some colours with ImageMagick but it makes wholesale changes to the palette and doesn't store it back in an identical fashion to the original image, so I doubt your game would load it.

Instead, you can dump the palette with ImageMagick like this:

identify -verbose palette.bmp

Output

Image: palette.bmp
  Format: BMP3 (Microsoft Windows bitmap image (V3))
  Class: PseudoClass
  Geometry: 23x23+0+0
  Resolution: 28.34x28.34
  Print size: 0.811574x0.811574
  Units: PixelsPerCentimeter
  Colorspace: sRGB
  Type: Palette
  Base type: Undefined
  Endianess: Undefined
  Depth: 8-bit
  Channel depth:
    Red: 8-bit
    Green: 8-bit
    Blue: 8-bit
  Channel statistics:
    Pixels: 529
    Red:
      min: 0  (0)
      max: 255 (1)
      mean: 147.877 (0.57991)
      standard deviation: 79.3755 (0.311277)
      kurtosis: -1.08138
      skewness: -0.716658
      entropy: 0.829558
    Green:
      min: 0  (0)
      max: 255 (1)
      mean: 137.873 (0.54068)
      standard deviation: 67.4838 (0.264642)
      kurtosis: -0.62805
      skewness: -0.58513
      entropy: 0.8385
    Blue:
      min: 0  (0)
      max: 255 (1)
      mean: 127.45 (0.499804)
      standard deviation: 68.6364 (0.269162)
      kurtosis: -0.63322
      skewness: -0.190579
      entropy: 0.842021
  Image statistics:
    Overall:
      min: 0  (0)
      max: 255 (1)
      mean: 137.733 (0.540131)
      standard deviation: 71.8319 (0.281694)
      kurtosis: -0.913123
      skewness: -0.483209
      entropy: 0.836693
  Colors: 65
  Histogram:
        32: (  0,  0,  0) #000000 black
        18: (  7,  7,  7) #070707 srgb(7,7,7)
         1: ( 13, 16, 41) #0D1029 srgb(13,16,41)
         1: ( 14, 14, 14) #0E0E0E srgb(14,14,14)
        44: ( 33, 65, 43) #21412B srgb(33,65,43)
        11: ( 38, 88, 71) #265847 srgb(38,88,71)
         3: ( 47, 94, 43) #2F5E2B srgb(47,94,43)
         3: ( 51, 51, 51) #333333 grey20
         7: ( 54, 56, 67) #363843 srgb(54,56,67)
        12: ( 58, 58, 58) #3A3A3A srgb(58,58,58)
         1: ( 58, 62, 74) #3A3E4A srgb(58,62,74)
         1: ( 63, 68, 80) #3F4450 srgb(63,68,80)
         2: ( 66, 66, 66) #424242 grey26
         1: ( 70, 70, 70) #464646 srgb(70,70,70)
         1: ( 73, 80, 93) #49505D srgb(73,80,93)
         2: ( 76, 76, 76) #4C4C4C srgb(76,76,76)
         2: ( 81, 81, 81) #515151 srgb(81,81,81)
         1: ( 82, 37, 20) #522514 srgb(82,37,20)
         1: ( 83, 93,105) #535D69 srgb(83,93,105)
         1: ( 86, 86, 86) #565656 srgb(86,86,86)
         1: ( 90, 60, 49) #5A3C31 srgb(90,60,49)
         5: ( 91, 91, 91) #5B5B5B srgb(91,91,91)
         1: ( 96, 96, 96) #606060 srgb(96,96,96)
         4: ( 98,112,125) #62707D srgb(98,112,125)
         1: (101,101,101) #656565 srgb(101,101,101)
         1: (106, 69, 66) #6A4542 srgb(106,69,66)
         1: (111,111,111) #6F6F6F srgb(111,111,111)
         2: (114,158,140) #729E8C srgb(114,158,140)
         1: (116,160,182) #74A0B6 srgb(116,160,182)
        11: (117, 89, 74) #75594A srgb(117,89,74)
         2: (117,117,117) #757575 grey46
         3: (117,182,232) #75B6E8 srgb(117,182,232)
         1: (121,142, 97) #798E61 srgb(121,142,97)
         9: (126,140,134) #7E8C86 srgb(126,140,134)
         1: (150,138,112) #968A70 srgb(150,138,112)
         2: (159,119,115) #9F7773 srgb(159,119,115)
         9: (165,148,142) #A5948E srgb(165,148,142)
        28: (167,167,177) #A7A7B1 srgb(167,167,177)
         1: (168,144,105) #A89069 srgb(168,144,105)
        10: (172,169,143) #ACA98F srgb(172,169,143)
        16: (172,198,213) #ACC6D5 srgb(172,198,213)
         6: (174,150,111) #AE966F srgb(174,150,111)
         4: (176,184,155) #B0B89B srgb(176,184,155)
         4: (181,156,116) #B59C74 srgb(181,156,116)
         6: (187,162,122) #BBA27A srgb(187,162,122)
         3: (192,228,231) #C0E4E7 srgb(192,228,231)
         5: (194,169,127) #C2A97F srgb(194,169,127)
         8: (195,128,148) #C38094 srgb(195,128,148)
         6: (195,190,182) #C3BEB6 srgb(195,190,182)
         1: (199,194,187) #C7C2BB srgb(199,194,187)
        36: (200,175,133) #C8AF85 srgb(200,175,133)
         1: (202,132,153) #CA8499 srgb(202,132,153)
        26: (207,181,138) #CFB58A srgb(207,181,138)
        34: (213,147,144) #D59390 srgb(213,147,144)
        16: (213,187,144) #D5BB90 srgb(213,187,144)
        13: (214,141,163) #D68DA3 srgb(214,141,163)
        30: (216,240,255) #D8F0FF srgb(216,240,255)
         7: (220,194,150) #DCC296 srgb(220,194,150)
         5: (221,145,168) #DD91A8 srgb(221,145,168)
         1: (223,218,212) #DFDAD4 srgb(223,218,212)
        35: (226,190,172) #E2BEAC srgb(226,190,172)
         3: (227,149,173) #E395AD srgb(227,149,173)
         2: (227,222,216) #E3DED8 srgb(227,222,216)
        21: (231,226,221) #E7E2DD srgb(231,226,221)
         1: (255,255,255) #FFFFFF white
  Colormap entries: 256
  Colormap:
         0: (255,255,255,255) #FFFFFFFF white
         1: (128,  0,  0,255) #800000FF maroon
         2: (  0,128,  0,255) #008000FF green
         3: (128,128,  0,255) #808000FF olive
         4: (  0,  0,128,255) #000080FF navy
         5: (128,  0,128,255) #800080FF purple
         6: (  0,128,128,255) #008080FF teal
         7: (192,192,192,255) #C0C0C0FF silver
         8: (192,204,216,255) #C0CCD8FF srgba(192,204,216,1)
         9: (  0,  0,  0,255) #000000FF black
        10: (231,226,221,255) #E7E2DDFF srgba(231,226,221,1)
        11: (227,222,216,255) #E3DED8FF srgba(227,222,216,1)     <--- LOOK HERE
        12: (223,218,212,255) #DFDAD4FF srgba(223,218,212,1)
        13: (219,214,208,255) #DBD6D0FF srgba(219,214,208,1)
        14: (215,210,204,255) #D7D2CCFF srgba(215,210,204,1)
        15: (211,206,199,255) #D3CEC7FF srgba(211,206,199,1)
        16: (207,202,195,255) #CFCAC3FF srgba(207,202,195,1)
        17: (203,198,191,255) #CBC6BFFF srgba(203,198,191,1)
        18: (199,194,187,255) #C7C2BBFF srgba(199,194,187,1)
        19: (195,190,182,255) #C3BEB6FF srgba(195,190,182,1)
        20: (117,117,117,255) #757575FF grey46
        21: (111,111,111,255) #6F6F6FFF srgba(111,111,111,1)
        22: (106,106,106,255) #6A6A6AFF srgba(106,106,106,1)
        23: (101,101,101,255) #656565FF srgba(101,101,101,1)
        24: ( 96, 96, 96,255) #606060FF srgba(96,96,96,1)
        25: ( 91, 91, 91,255) #5B5B5BFF srgba(91,91,91,1)
        26: ( 86, 86, 86,255) #565656FF srgba(86,86,86,1)
        27: ( 81, 81, 81,255) #515151FF srgba(81,81,81,1)
        28: ( 76, 76, 76,255) #4C4C4CFF srgba(76,76,76,1)
        29: ( 70, 70, 70,255) #464646FF srgba(70,70,70,1)
        30: ( 66, 66, 66,255) #424242FF grey26
        31: ( 58, 58, 58,255) #3A3A3AFF srgba(58,58,58,1)
        32: ( 51, 51, 51,255) #333333FF grey20
        33: ( 44, 44, 44,255) #2C2C2CFF srgba(44,44,44,1)
        34: ( 36, 36, 36,255) #242424FF grey14
        35: ( 29, 29, 29,255) #1D1D1DFF srgba(29,29,29,1)
        36: ( 22, 22, 22,255) #161616FF srgba(22,22,22,1)
        37: ( 14, 14, 14,255) #0E0E0EFF srgba(14,14,14,1)
        38: (  7,  7,  7,255) #070707FF srgba(7,7,7,1)
        39: (  0,  0,  0,255) #000000FF black
        40: (220,194,150,255) #DCC296FF srgba(220,194,150,1)
        41: (213,187,144,255) #D5BB90FF srgba(213,187,144,1)
        42: (207,181,138,255) #CFB58AFF srgba(207,181,138,1)
        43: (200,175,133,255) #C8AF85FF srgba(200,175,133,1)
        44: (194,169,127,255) #C2A97FFF srgba(194,169,127,1)
        45: (187,162,122,255) #BBA27AFF srgba(187,162,122,1)
        46: (181,156,116,255) #B59C74FF srgba(181,156,116,1)
        47: (174,150,111,255) #AE966FFF srgba(174,150,111,1)
        48: (168,144,105,255) #A89069FF srgba(168,144,105,1)
        49: (162,137, 99,255) #A28963FF srgba(162,137,99,1)
        50: (135, 65, 34,255) #874122FF srgba(135,65,34,1)
        51: (127, 61, 32,255) #7F3D20FF srgba(127,61,32,1)
        52: (119, 57, 30,255) #77391EFF srgba(119,57,30,1)
        53: (112, 53, 28,255) #70351CFF srgba(112,53,28,1)
        54: (104, 49, 26,255) #68311AFF srgba(104,49,26,1)
        55: ( 97, 45, 24,255) #612D18FF srgba(97,45,24,1)
        56: ( 89, 41, 22,255) #592916FF srgba(89,41,22,1)
        57: ( 82, 37, 20,255) #522514FF srgba(82,37,20,1)
        58: ( 74, 33, 18,255) #4A2112FF srgba(74,33,18,1)
        59: ( 66, 29, 16,255) #421D10FF srgba(66,29,16,1)
        60: (180,115, 22,255) #B47316FF srgba(180,115,22,1)
        61: (175,109, 19,255) #AF6D13FF srgba(175,109,19,1)
        62: (170,104, 17,255) #AA6811FF srgba(170,104,17,1)
        63: (165, 99, 14,255) #A5630EFF srgba(165,99,14,1)
        64: (161, 94, 12,255) #A15E0CFF srgba(161,94,12,1)
        65: (156, 88,  9,255) #9C5809FF srgba(156,88,9,1)
        66: (151, 83,  7,255) #975307FF srgba(151,83,7,1)
        67: (147, 78,  4,255) #934E04FF srgba(147,78,4,1)
        68: (142, 73,  2,255) #8E4902FF srgba(142,73,2,1)
        69: (137, 68,  0,255) #894400FF srgba(137,68,0,1)
        70: (240,158,183,255) #F09EB7FF srgba(240,158,183,1)
        71: (233,153,178,255) #E999B2FF srgba(233,153,178,1)
        72: (227,149,173,255) #E395ADFF srgba(227,149,173,1)
        73: (221,145,168,255) #DD91A8FF srgba(221,145,168,1)
        74: (214,141,163,255) #D68DA3FF srgba(214,141,163,1)
        75: (208,136,158,255) #D0889EFF srgba(208,136,158,1)
        76: (202,132,153,255) #CA8499FF srgba(202,132,153,1)
        77: (195,128,148,255) #C38094FF srgba(195,128,148,1)
        78: (189,124,143,255) #BD7C8FFF srgba(189,124,143,1)
        79: (183,119,139,255) #B7778BFF srgba(183,119,139,1)
        80: (168, 41,  1,255) #A82901FF srgba(168,41,1,1)
        81: (164, 40,  1,255) #A42801FF srgba(164,40,1,1)
        82: (159, 39,  1,255) #9F2701FF srgba(159,39,1,1)
        83: (155, 38,  1,255) #9B2601FF srgba(155,38,1,1)
        84: (151, 37,  1,255) #972501FF srgba(151,37,1,1)
        85: (146, 36,  1,255) #922401FF srgba(146,36,1,1)
        86: (142, 35,  1,255) #8E2301FF srgba(142,35,1,1)
        87: (138, 34,  1,255) #8A2201FF srgba(138,34,1,1)
        88: (133, 33,  1,255) #852101FF srgba(133,33,1,1)
        89: (129, 32,  1,255) #812001FF srgba(129,32,1,1)
        90: (107, 74, 12,255) #6B4A0CFF srgba(107,74,12,1)
        91: (101, 68, 11,255) #65440BFF srgba(101,68,11,1)
        92: ( 96, 62, 11,255) #603E0BFF srgba(96,62,11,1)
        93: ( 91, 57, 11,255) #5B390BFF srgba(91,57,11,1)
        94: ( 86, 51, 11,255) #56330BFF srgba(86,51,11,1)
        95: ( 81, 45, 10,255) #512D0AFF srgba(81,45,10,1)
        96: ( 76, 39, 10,255) #4C270AFF srgba(76,39,10,1)
        97: ( 71, 34, 10,255) #47220AFF srgba(71,34,10,1)
        98: ( 66, 28, 10,255) #421C0AFF srgba(66,28,10,1)
        99: ( 60, 22,  9,255) #3C1609FF srgba(60,22,9,1)
       100: (166,138, 56,255) #A68A38FF srgba(166,138,56,1)
       101: (162,133, 55,255) #A28537FF srgba(162,133,55,1)
       102: (158,129, 55,255) #9E8137FF srgba(158,129,55,1)
       103: (154,125, 55,255) #9A7D37FF srgba(154,125,55,1)
       104: (150,120, 54,255) #967836FF srgba(150,120,54,1)
       105: (147,116, 54,255) #937436FF srgba(147,116,54,1)
       106: (143,112, 54,255) #8F7036FF srgba(143,112,54,1)
       107: (139,107, 53,255) #8B6B35FF srgba(139,107,53,1)
       108: (135,103, 53,255) #876735FF srgba(135,103,53,1)
       109: (132, 98, 53,255) #846235FF srgba(132,98,53,1)
       110: ( 98,112,125,255) #62707DFF srgba(98,112,125,1)
       111: ( 93,105,118,255) #5D6976FF srgba(93,105,118,1)
       112: ( 88, 99,112,255) #586370FF srgba(88,99,112,1)
       113: ( 83, 93,105,255) #535D69FF srgba(83,93,105,1)
       114: ( 78, 87, 99,255) #4E5763FF srgba(78,87,99,1)
       115: ( 73, 80, 93,255) #49505DFF srgba(73,80,93,1)
       116: ( 68, 74, 86,255) #444A56FF srgba(68,74,86,1)
       117: ( 63, 68, 80,255) #3F4450FF srgba(63,68,80,1)
       118: ( 58, 62, 74,255) #3A3E4AFF srgba(58,62,74,1)
       119: ( 54, 56, 67,255) #363843FF srgba(54,56,67,1)
       120: (154,142,115,255) #9A8E73FF srgba(154,142,115,1)
       121: (150,138,112,255) #968A70FF srgba(150,138,112,1)
       122: (147,135,109,255) #93876DFF srgba(147,135,109,1)
       123: (144,132,107,255) #90846BFF srgba(144,132,107,1)
       124: (140,129,104,255) #8C8168FF srgba(140,129,104,1)
       125: (137,126,102,255) #897E66FF srgba(137,126,102,1)
       126: (134,123, 99,255) #867B63FF srgba(134,123,99,1)
       127: (130,120, 97,255) #827861FF srgba(130,120,97,1)
       128: (127,117, 94,255) #7F755EFF srgba(127,117,94,1)
       129: (124,113, 91,255) #7C715BFF srgba(124,113,91,1)
       130: ( 85,171, 87,255) #55AB57FF srgba(85,171,87,1)
       131: ( 60,161, 71,255) #3CA147FF srgba(60,161,71,1)
       132: ( 21,153, 23,255) #159917FF srgba(21,153,23,1)
       133: ( 53,131, 54,255) #358336FF srgba(53,131,54,1)
       134: ( 48,123, 28,255) #307B1CFF srgba(48,123,28,1)
       135: ( 16,121, 25,255) #107919FF srgba(16,121,25,1)
       136: ( 39, 98, 23,255) #276217FF srgba(39,98,23,1)
       137: ( 47, 94, 43,255) #2F5E2BFF srgba(47,94,43,1)
       138: ( 19, 92, 20,255) #135C14FF srgba(19,92,20,1)
       139: ( 16, 65, 17,255) #104111FF srgba(16,65,17,1)
       140: ( 43, 97,195,255) #2B61C3FF srgba(43,97,195,1)
       141: ( 56, 70,227,255) #3846E3FF srgba(56,70,227,1)
       142: ( 51, 59,255,255) #333BFFFF srgba(51,59,255,1)
       143: ( 51, 67,206,255) #3343CEFF srgba(51,67,206,1)
       144: ( 22, 26,215,255) #161AD7FF srgba(22,26,215,1)
       145: ( 46, 60,182,255) #2E3CB6FF srgba(46,60,182,1)
       146: ( 22, 28,169,255) #161CA9FF srgba(22,28,169,1)
       147: ( 42, 66,144,255) #2A4290FF srgba(42,66,144,1)
       148: ( 25, 34,119,255) #192277FF srgba(25,34,119,1)
       149: ( 17, 25, 83,255) #111953FF srgba(17,25,83,1)
       150: (216,240,255,255) #D8F0FFFF srgba(216,240,255,1)
       151: (172,224,255,255) #ACE0FFFF srgba(172,224,255,1)
       152: (153,214,255,255) #99D6FFFF srgba(153,214,255,1)
       153: (130,202,255,255) #82CAFFFF srgba(130,202,255,1)
       154: (117,182,232,255) #75B6E8FF srgba(117,182,232,1)
       155: (104,192,255,255) #68C0FFFF srgba(104,192,255,1)
       156: ( 81,150,220,255) #5196DCFF srgba(81,150,220,1)
       157: ( 24,156,206,255) #189CCEFF srgba(24,156,206,1)
       158: ( 86,139,185,255) #568BB9FF srgba(86,139,185,1)
       159: ( 30,137,169,255) #1E89A9FF srgba(30,137,169,1)
       160: (235,237,167,255) #EBEDA7FF srgba(235,237,167,1)
       161: (234,235,144,255) #EAEB90FF srgba(234,235,144,1)
       162: (209,204,119,255) #D1CC77FF srgba(209,204,119,1)
       163: (247,244,  0,255) #F7F400FF srgba(247,244,0,1)
       164: (237,232, 50,255) #EDE832FF srgba(237,232,50,1)
       165: (195,196, 11,255) #C3C40BFF srgba(195,196,11,1)
       166: (195,195, 52,255) #C3C334FF srgba(195,195,52,1)
       167: (159,162, 23,255) #9FA217FF srgba(159,162,23,1)
       168: (159,159, 67,255) #9F9F43FF srgba(159,159,67,1)
       169: (110,122, 46,255) #6E7A2EFF srgba(110,122,46,1)
       170: (255,255,255,255) #FFFFFFFF white
       171: (192,228,231,255) #C0E4E7FF srgba(192,228,231,1)
       172: (172,198,213,255) #ACC6D5FF srgba(172,198,213,1)
       173: (167,167,177,255) #A7A7B1FF srgba(167,167,177,1)
       174: (160,160,168,255) #A0A0A8FF srgba(160,160,168,1)
       175: (116,160,182,255) #74A0B6FF srgba(116,160,182,1)
       176: (131,153,177,255) #8399B1FF srgba(131,153,177,1)
       177: (131,153,177,255) #8399B1FF srgba(131,153,177,1)
       178: (128,152,176,255) #8098B0FF srgba(128,152,176,1)
       179: (128,152,176,255) #8098B0FF srgba(128,152,176,1)
       180: (226,190,172,255) #E2BEACFF srgba(226,190,172,1)
       181: (213,147,144,255) #D59390FF srgba(213,147,144,1)
       182: (215,118,110,255) #D7766EFF srgba(215,118,110,1)
       183: (184,115,103,255) #B87367FF srgba(184,115,103,1)
       184: (159,119,115,255) #9F7773FF srgba(159,119,115,1)
       185: (162,106, 93,255) #A26A5DFF srgba(162,106,93,1)
       186: (137,100, 88,255) #896458FF srgba(137,100,88,1)
       187: (152, 87, 77,255) #98574DFF srgba(152,87,77,1)
       188: (106, 69, 66,255) #6A4542FF srgba(106,69,66,1)
       189: ( 90, 60, 49,255) #5A3C31FF srgba(90,60,49,1)
       190: (114,158,140,255) #729E8CFF srgba(114,158,140,1)
       191: (  0,128,128,255) #008080FF teal
       192: ( 66,122,117,255) #427A75FF srgba(66,122,117,1)
       193: (  0,128,128,255) #008080FF teal
       194: ( 57,120, 96,255) #397860FF srgba(57,120,96,1)
       195: ( 61, 90, 99,255) #3D5A63FF srgba(61,90,99,1)
       196: ( 38, 88, 71,255) #265847FF srgba(38,88,71,1)
       197: ( 33, 65, 43,255) #21412BFF srgba(33,65,43,1)
       198: ( 18, 57, 48,255) #123930FF srgba(18,57,48,1)
       199: (255,255,255,255) #FFFFFFFF white
       200: (255,255,255,255) #FFFFFFFF white
       201: (244,246,216,255) #F4F6D8FF srgba(244,246,216,1)
       202: (233,216,194,255) #E9D8C2FF srgba(233,216,194,1)
       203: ( 44, 95, 89,255) #2C5F59FF srgba(44,95,89,1)
       204: (211,244,197,255) #D3F4C5FF srgba(211,244,197,1)
       205: (196,211,159,255) #C4D39FFF srgba(196,211,159,1)
       206: (255,199, 26,255) #FFC71AFF srgba(255,199,26,1)
       207: (176,184,155,255) #B0B89BFF srgba(176,184,155,1)
       208: (175,177,119,255) #AFB177FF srgba(175,177,119,1)
       209: (165,148,142,255) #A5948EFF srgba(165,148,142,1)
       210: (172,169,143,255) #ACA98FFF srgba(172,169,143,1)
       211: (215,160, 20,255) #D7A014FF srgba(215,160,20,1)
       212: (198,127,  8,255) #C67F08FF srgba(198,127,8,1)
       213: (202,110, 70,255) #CA6E46FF srgba(202,110,70,1)
       214: (121,142, 97,255) #798E61FF srgba(121,142,97,1)
       215: (153,126, 77,255) #997E4DFF srgba(153,126,77,1)
       216: (128,128,128,255) #808080FF fractal
       217: (101,155, 42,255) #659B2AFF srgba(101,155,42,1)
       218: (  0,203, 22,255) #00CB16FF srgba(0,203,22,1)
       219: (167,108, 57,255) #A76C39FF srgba(167,108,57,1)
       220: (255, 66,  0,255) #FF4200FF srgba(255,66,0,1)
       221: (151,100, 66,255) #976442FF srgba(151,100,66,1)
       222: (153,101, 42,255) #99652AFF srgba(153,101,42,1)
       223: (221, 52,106,255) #DD346AFF srgba(221,52,106,1)
       224: ( 41, 68,117,255) #294475FF srgba(41,68,117,1)
       225: ( 88,105,181,255) #5869B5FF srgba(88,105,181,1)
       226: ( 66,107,132,255) #426B84FF srgba(66,107,132,1)
       227: ( 80,100,128,255) #506480FF srgba(80,100,128,1)
       228: (117, 89, 74,255) #75594AFF srgba(117,89,74,1)
       229: (254, 24, 20,255) #FE1814FF srgba(254,24,20,1)
       230: (110, 92, 43,255) #6E5C2BFF srgba(110,92,43,1)
       231: (156, 66, 57,255) #9C4239FF srgba(156,66,57,1)
       232: (184, 50, 66,255) #B83242FF srgba(184,50,66,1)
       233: (136, 68, 66,255) #884442FF srgba(136,68,66,1)
       234: ( 85, 68,127,255) #55447FFF srgba(85,68,127,1)
       235: (126,140,134,255) #7E8C86FF srgba(126,140,134,1)
       236: (207, 16, 12,255) #CF100CFF srgba(207,16,12,1)
       237: (146, 25, 20,255) #921914FF srgba(146,25,20,1)
       238: ( 51, 47, 18,255) #332F12FF srgba(51,47,18,1)
       239: ( 17, 40, 14,255) #11280EFF srgba(17,40,14,1)
       240: ( 13, 16, 41,255) #0D1029FF srgba(13,16,41,1)
       241: (  0,  0,  0,255) #000000FF black
       242: (  0,  0,  0,255) #000000FF black
       243: (  0,  0,  0,255) #000000FF black
       244: (  0,  0,  0,255) #000000FF black
       245: (255,255,255,255) #FFFFFFFF white
       246: ( 80,100,128,255) #506480FF srgba(80,100,128,1)
       247: (  0,  0,  0,255) #000000FF black
       248: (128,128,128,255) #808080FF fractal
       249: (255,  0,  0,255) #FF0000FF red
       250: (  0,255,  0,255) #00FF00FF lime
       251: (255,255,  0,255) #FFFF00FF yellow
       252: (  0,  0,255,255) #0000FFFF blue
       253: (255,  0,255,255) #FF00FFFF magenta
       254: (  0,255,255,255) #00FFFFFF cyan
       255: (  0,  0,  0,255) #000000FF black
  Rendering intent: Perceptual
  Gamma: 0.454545
  Chromaticity:
    red primary: (0.64,0.33)
    green primary: (0.3,0.6)
    blue primary: (0.15,0.06)
    white point: (0.3127,0.329)
  Matte color: grey74
  Background color: white
  Border color: srgb(223,223,223)
  Transparent color: none
  Interlace: None
  Intensity: Undefined
  Compose: Over
  Page geometry: 23x23+0+0
  Dispose: Undefined
  Iterations: 0
  Compression: None
  Orientation: Undefined
  Properties:
    date:create: 2018-04-05T08:00:58+01:00
    date:modify: 2018-04-05T07:59:56+01:00
    signature: e403caa8a487c57943e3cdce3370485abb3ce54e045b0342a9e5826f5a9fe3ad
  Artifacts:
    verbose: true
  Tainted: False
  Filesize: 1630B
  Number pixels: 529
  User time: 0.000u
  Elapsed time: 0:01.000
  Version: ImageMagick 7.0.7-28 Q16 x86_64 2018-03-27 http://www.imagemagick.org

I have annotated that above with <--- LOOK HERE where the palette (colormap) entries are shown.

If you load the file into a hex editor, such as the online one here you can find that entry and the ones around it and diddle with them to your heart's content!

I have marked in red the corresponding entry in the palette. Note the palette entries all have a superfluous FF for transparency in ImageMagick so you won't see that FF in the hex.

So, let's make a swatch of that colour (#E3DED8):

convert -size 100x40 xc:"#E3DED8" swatch.gif

And replace it with magenta in the hex dump and see what happens: