BLACK
Is the most common of all Rat Terrier colors. A black and white dog with tan point markings is often referred to as the CLASSIC TRI as it is considered by many to be the typical Rat Terrier color and a common favorite. Blacks have black noses and black eye rims.

TAN / TAN SABLE
Is also a common Rat Terrier color. Tan may vary from very light buckskin to deep mahogany red, often darkening with age. Black sable hairs are common on tan dogs and are typically dense at birth and lessen with maturity (see page 3: SABLE) Tan is not a dilute color as all tan dogs have black noses and eye rims.

CHOCOLATE
Was once considered unusual, but is now a common Rat Terrier color. Chocolates vary from from dark, liver chocolate to very light, reddish chocolate, often referred to as High Red Chocolate. The definitive feature of a Chocolate Rat Terrier is the chocolate (non-black) nose. Chocolate may be called Liver or Red in other breeds. Chocolate is not a dilute but a restrictive color as the dog cannot produce black pigment or black hairs. Tan puppies occasionally appear chocolate at birth but if the nose color fills in black, the puppy is tan.

BLUE

Is a true dilute and appears as gray or gray-blue on the dog. Blue is called Maltese Blue in some breeds and is genetically a black dog with the dilute gene affecting the black color at a cellular level. The nose color is generally dark blue and adult eye color may be dark blue, golden or hazel. Light blue puppy eyes usually turn amber with maturity. A condition of baldness called CDA (Color Dilution Alopecia) is a problem in some breeds that have dilutes and it is believed that risks of CDA increase when dilutes are bred to each other. As yet, CDA is not a common problem in Rat Terriers, but avoiding the breeding of dilutes to dilutes is recommended and seems to be a wise precaution.

 

Genetic Study found here: http://www.1st-writer.com/ColorGenetics/lesson1.htm