Compared to colorful fish, lizards, birds, and insects, we mammals are downright drab:
Unless you are a color scientist you are probably accustomed to dealing with chemical colors. For example, if you take a handful of blue pigment powder, mix it with water, paint it onto a chair, let it dry, then scrape it off the chair, and grind it back into powder, you expect it to remain blue at all stages in the process (except if you get a bit of chair mixed in with it.)
By contrast, if you scraped the scales off a blue morpho butterfly’s wings, you’d just end up with a pile of grey dust and a sad butterfly. By themselves, blue morpho scales are not “blue,” even under regular light. Rather, their scales are arranged so that light bounces between them, like light bouncing from molecule to molecule in the air.
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This kind of structural color works great if your medium is scales, feathers, carapaces, berries, or even CDs, but just doesn’t work with hair, which we mammals have.
Compared to other animals, mammals also have bad color perception, which may be explained by the nocturnal bottleneck hypothesis:
The hypothesis states that mammals were mainly or even exclusively nocturnal through most of their evolutionary story, starting with their origin 225 million years ago, and only ending with the demise of the dinosaurs 65 millions years ago. While some mammal groups have later evolved to fill diurnal niches, the 160 million years spent as nocturnal animals has left a lasting legacy on basal anatomy and physiology, and most mammals are still nocturnal.
Most of those small mammals too were burrowing and hibernating animals for much of the rest of the time when not foraging at night. That has some consequence too?