They are dye kit colors. All of the dye kit colors are: Amber, Azure, Gold & Silver (or Gold for short), Indigo, Lemon, Lime, Magenta, Orchid, Rose

Dye kits can be purchased from the Black Market shop, or from other users

The natural, or "hatchable" colors are: Natural, Sepia, Cream, Black, Ginger, Silver, Blonde, and Albino (listed from most common to least common)

In addition to that, Credit Shop (CS) creatu can ony be hatched as Natural, Black, or Albino, while "findable" eggs can be hatched in any natural color. Some creatu are not yet available in all dye kit colors, but the artists are working on updating them :)