Color Change

I have an Alaskan Malamute who is 9 years old.  He has 2 ACL surgeries and has alot of artheritis so he take Deramax and tramadol just to give you some back ground on him..his name is Bo.  My question though is in the last 2 months or so one side of his muzzle/cheek (his left) has turned a cocoa color, which of course has always been white.  He lays alot with his face resting on his 2 front legs, so then his corresponding left leg has a brown band on it and he chews at his leg in that area.  I try washing his face, and he gets groomed so I know it is not like a stain I just don't know what could cause something like this???

[Aleda M Cheng, D.V.M., C.V.A]

This is a very unusual question and I am not sure I have definitive answer for you, but I have some ideas as follows:

Porphyrin, a pigment found in the saliva and tears of dogs is a reddish brown color and can semi-permanently stain white hair.

Nutritional deficiencies can induce color change.  Please view the following scientific article from "The Journal of Nutrition":

http://jn.nutrition.org/cgi/content/full/134/8/2053S

The article is rather technical but it does list the amino acids responsible for pigment in hair and how the absence of some pigments can change the hair color.