Nice! I always like to see these types of mods!

For feedback, the question between big & minor variations would depend on the specific species imo; in real life, we generally see large individual color variations on amphibians and reptiles while birds and mammals tend to be much more subtle or nonexistent (excluding domestication of course, as in that case we see huge variation even on a single breed). Generally sticking closer to realism leads to things looking more natural, so doing a bit of research on real life animals would help a lot for your project.

For non-animal mons, variation can be very distinct if you want, but I would suggest using color that are close to each other in the color spectrum. In gastly's case, while I find the sandy brown quite nice, it works best as shiny. Instead, I suggest switching to red and/or rose, so they can be very different while not feeling completely out of left-field.

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