There are many different dilution and modifier genes that can affect coat color (and sometimes skin pigmentation) in dogs. These include Brown (liver), Dilution (the D locus), Dilution of Red (Pheomelanin), Urajiro, Graying, and Merle.
Sunset Acres 50 Cent is a wonderful example of a black and tan rat terrier whose black areas have been diluted to brown. Notice the intensity of the red (tan) pigment is not reduced. Photo courtesy of rodeoratdogs
A dominant black dog known to be KBky, as his sire was not black or brindle
Colors: Black, Yellow, and Red
Canines produce two types of pigment: eumelanin, which is black pigment; and phaeomelanin which is red/yellow pigment. It is the combination of these pigments, along with dilutions, modifiers, and white patterns, that make up the entire spectrum of dog coat colors.
In dogs and wolves there are four known alleles of the extension locus: EM, EG, E, and e. In the absence of dilutions or white patterns, a dog that is ee will be red or yellow. Its skin and eyes will still be dark, but the coat will only contain red or yellow pigment; thereby producing a red or yellow dog. In an ee dog, many other genes are masked. They are there, and will still be passed on to offspring, but they will not express. Dilutions and white patterns will still affect an ee dog.