How coat color is being inherited in these horses.

A black mare was crossed to a chestnut stallion and produced a bay son and a bay daughter.  The two offspring were mated to each other several times, and they produced offspring of four different coat colors:  black, bay, chestnut, and liver.  Crossing a liver grandson back to the black mare gave a black foal, and crossing a liver granddaughter back to the chestnut stallion gave a chestnut foal.  Explain how coat color is being inherited in these horses. 

i think it is via epistasis, because the black and the allele and the chestnut allele mask the phenotype of the liver color?

am i correct


thanks