THe rarer the color, the more people will pay. You can figure this out by looking up the creatu in the hatchery's creatu list. Natural is the most common, and it goes all the way down to albino, which is the hardest to hatch. Then there are died creatu, which can be colored by buying die kits from the black market. THis is allways extreemly expencive, and up the creatu's value.
The name does, too. Some names, like most of my pets, have names with sentamental value and are truely important only to the owner. THis is allso why unnamed creatu are occasionaly popular, depening, again, on the color. Usualy, RWN, or Real World Names, sell big. This includes creatu names that are real words or have real life meanings.
You can also search other people's creatu shops in the seach bar at the top of the page. THis helps you know what other people are paying for a certin kind of name, color, or rarety. (Yes, the rarer the creatu is, the higher its value usually is.) Some people are attracted to a certain creatu, and some just buy what they like. Anyway, shops are a great way to base your own prices off of.
Hope this helps!
~~Morgain~~~