The dorsal fur in yellow F1 mice (F1-Ay) between C3H/HeJ and C57BL/6J-Ay is darker than that in C57BL/6J-Ay. Moreover, yellow F2 mice (F2-Ay) exhibit a wide spectrum of coat color phenotypes in terms of lightness and darkness. Quantitative trait locus (QTL) analysis on F2-Ay identified three significant modifier loci that accounted for darkening of the coat color on chromosomes 1 (Dmyaq1 and Dmyaq2) and 15 (Dmyaq3), and the C3H/HeJ allele at these loci increased the darkness. Because agouti F2 mice (F2-A) also exhibited a spectrum of coat color phenotypes, the question of whether these QTLs had any effects on F2-A was examined. Dmyaq1 and Dmyaq2 were shown to increase the darkness in F2-A, whereas Dmyaq3 did not. The results showed that Dmyaq1–Dmyaq3 were parts of determinants responsible for the sable (darker modification of yellow) coat color phenotype, and that Dmyaq1 and Dmyaq2 were parts of determinants responsible for the umbrous (darker modification of agouti) coat color phenotype. It is, thus, demonstrated that both the sable and the umbrous phenotypes resulted from multigenic contributions, and that they shared genetic bases, as had been implied for several decades.