Hi Mike
If it is the hair of your dog's coat that is discoloured, then it could be staining that has caused it. There is a product to help prevent this called Angel Glow. It will not make the brownish colour disappear but will help prevent it from growning back in.
Where is the discolouration? If it is round the eyes and mouth then the groomer will cut it away.
Westies are prone to this.