Reading up on the MC1R, I came across this nice summary of work I mentioned before on the genetics of coat color in the dog. The summary includes the figure below, which is a pretty intuitive illustration of a few of the genes involved in pigment type-switching (ie. production of eumelanin versus pheomelanin). The caption:
Production of yellow versus black pigment in dogs is controlled by three genes: Mc1r, Agouti, and CBD103. Dogs carrying wild-type alleles for all three genes have a yellow coat resulting from Agouti antagonism of Mc1r signaling in melanocytes (yellow Great Dane, top). Dogs carrying a loss-of-function mutation at Mc1r have a yellow coat, regardless of their genotype at Agouti or CBD103 (yellow Labrador Retriever, middle). Dogs carrying wild-type alleles for Mc1r and Agouti, together with the dominant black allele of CBD103 (KB) have a black coat resulting from the interaction between a beta-defensin and Mc1r (black Curly Coated Retriever, bottom).