Trichromacy vs. dichromacy

Not sure I trust the image, but the principle appears to be valid.

Until the 1960s, popular belief held that most mammals outside of primates were monochromats. In the last half-century, however, a focus on behavioral and genetic testing of mammals has accumulated extensive evidence of dichromatic color vision in a number of mammalian orders. Mammals are now usually assumed to be dichromats (possessing S- and L-cones), with monochromats viewed as the exceptions.

The common vertebrate ancestor, extant during the Cambrian, was tetrachromatic, possessing 4 distinct opsins classes. Early mammalian evolution would see the loss of two of these four opsins, due to the nocturnal bottleneck, as dichromacy may improve an animal's ability to distinguish colors in dim light. Placental mammals are therefore – as a rule – dichromatic.

The exceptions to this rule of dichromatic vision in placental mammals are old world monkeys and apes, which re-evolved trichromacy, and marine mammals (both pinnipeds and cetaceans) which are cone monochromats. New World Monkeys are a partial exception: in most species, males are dichromats, and about 60% of females are trichromats, but the owl monkeys are cone monochromats, and both sexes of howler monkeys are trichromats.

Trichromacy has been retained or re-evolved in marsupials, where trichromatic vision is widespread. Recent genetic and behavioral evidence suggests the South American marsupial Didelphis albiventris is dichromatic, with only two classes of cone opsins having been found within the genus Didelphis.

Excerpt from the dichromacy entry of Wikipedia (see also trichromacy).  I didn't realize this.  You learn something every day.