I'm kind of new here so I don't know if people are suggesting new colors all the time and every gets annoyed, but...
I was thinking it would be cool if there was a hatchable color called "variegated." Which basically means having markings instead of a solid color. The term variegated allows for flexibility between stripes, spots, splotches, speckles, etc.--because some Creatu (mirabilis, gondra?) might look better in stripes, and some (zenirix, tesuri?) might look better in spots, or pied--and because some already have stripes or patches.
As for the specific hues, maybe something close to the species' "natural"color, but perhaps more subdued, or darkened if it's a pale color? I'm thinking of canaries; the classic solid yellow vs. the variegated which is more brown or even green.
And there might be a catchier word to use than "variegated" but I can't think of one at the moment...