Dichromacy

In dichromacy, the unique hues can be evoked by exciting only a single cone at a time, e.g. monochromatic light near the extremes of the visible spectrum.

Unlike trichromats, white (experienced when both cone cells are equally excited) can be evoked by monochromatic light.

Normal human color vision is trichromatic, so dichromacy is achieved by losing functionality of one of the three cone cells.

Furthermore, dichromats have a significantly higher threshold than trichromats for colored stimuli flickering at low (1 Hz) frequencies.

[8][9] This means such animals would still observe the flicker instead of a temporally fused visual perception as is the case in human movie watching at a high enough frame rate.

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.

The common vertebrate ancestor, extant during the Cambrian, was tetrachromatic, possessing 4 distinct opsins classes.

[6] 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.