В Википэдии очень интересная дефиниция цвета, но в языке английском
Fawn in animal breeds
The fawn colour is the result of two recessive genes. It is achieved by breeding dogs that one way or another do carry the dilution gene either in its homozygous or heterozygous form, that is the "D" and the chocolate gene "B". Black to black or black to red, red to red or black to blue, or even blue to red colour can produce a fawn coloured offspring, but only if the genes pair correctly and if they are both carrying the recessive form of the "B" and "D" gene.
Dog Breeds
The Breeds that are known for fawn coats include Pugs and Chihuahuas. Though some breeds have coats of similar or identical colour (such as yellow Labrador Retrievers), the dog must have a black "mask" on the mouth to be considered fawn.
http://en.wikipedia.org/wiki/Fawn_%28colour%29