What are the rarest color's for each pet

Well, that depends what you mean by rarest. The hardest to breed or the one that is actually around the site the least?

Hardest to breed is always the most recessive colour - you can see that in Help -> List of Skins. Example Unicorns' most recessive is Cream and Gray. So this means that a Silver Gray is in theory the rarest colour, since it's hardest to breed (both Gray and Cream have the least Power of all the colour genes, so whenever you add ANY other colour gene in there, that one will always dominate the other). If it's also the rarest colour around? This of course depends on how many people breed specificlaly to get that colour. But yeah, I guess usually the less dominant colours are also the rarer ones. 

Take Pumas as example - White is the colour gene with the least Power and it is indeed very hard to find a Puma out there who has it at least recessive. Most of the ones who are around have Chocolate Moose and/or Kai-Wen multiple times in their lineage. Just take this Puma as example: http://beastkeeper.com/pet/214877 - if you do a search in her Pedigree, she has Kai-Wen in there 6 times and Chocolate Moose 16 times!

Thanks!