The first mammals were small burrowing animals that climbed out at night, so as not to get caught by a dinosaur. Vision, distinguishing colors well, they did not need. And if there is a problem with color vision (most mammals do not see green and red), then a multi-colored color will not, for example, be an element of sexual attractiveness and will not give advantages in increasing offspring (birds with color vision are much better). Therefore, the color in mammals performs only the role of disguise, helping the predator to hide, and its prey - to merge with the landscape and be saved. Color vision, close to ours, appears only among primates, and species among the monkeys already have multi-colored faces, red backs and even blue testicles.
The article “Readers' Questions” was published in the journal Popular Mechanics (No. 6, June 2019).