A Chocolate rat should be a very deep, rich brown, very nearly a black. Think "dark" or baking chocolate, or black coffee with no cream, and you have an idea of the color. Ideally, there should be no silvering or any patchyness in the color. The color shouldn't be a brown that is mistaken for Mink - it should be so dark that it could be mistaken for Black, if anything.

a/a b/b

The color gene for Chocolate, which is "Brown," was discovered in England and important to the United States; however, very few "Chocolates" that exist currently in the country seem to breed true. They are usually very dark Mink carrying no other recessives, or very dark UK Minks - when bred to Mink rats, they do not throw Black babies. Chocolate is supposed to work like Mink in that it can be acted upon by Red to produce "Cocoa," which is equivalent to Havana.

AFRMA

"A deep rich chocolate, as even as possible, devoid of dinginess and white hairs or patches. The feet to match top color. Eyes black."

RSA

"Color: To be a deep rich chocolate, as even as possible, devoid of any dinginess.  Foot color to match top.

Faults: Patchiness.

Eye color: Black"

RatsPacNW

"A warm rich brown to skin with black eyes, may have black or dark tipped guard hairs."