I tried following Godot's documentation on MultiMeshes (the set_instance_color is specially interesting), but my models always end up washed out, way brighter than they should be (left is the correct model, right is after multimeshes applied):
It's a model made out of voxels. Each cube is put into the same multimesh and then each instance's color is changed depending on the imported model's colors.

I fixed it by setting
vertex_color_is_srgbtotruein my material.