Converting all images to RGB is quite handful to make sure all uploaded images are displayed in this specific color format.
Using it through imagemagick, the following :
-colorspace RGB
seems to be the option to pass.
However, the images processed with this option are quite darkened, which is a problem. Is there another option to pass to make sure the conversion preserve the colors used ?
I think you probably want to use
-colorspace sRGBsince that is actually the colourspace of the Web, if there is such a thing.