I’m not understanding this concern.
This movie is supposed to look more realistic, the characters resembling equines more so than their G4 counterparts. We can see their fur in detail, hooves that actually shine in light, inner color of their ears, etc. Making them bright/crazy colors wouldn’t mix well with the realism they are going for. Like @EpicEnergymentioned, this choice may also because more unique colors are just not needed anymore.
If the colors of the characters are going to have an influence on an overall opinion on this movie or G5 in general, I gotta say that’s pretty absurd.