There is no green pigment nor is there blue pigment for mammals. Mammals only have melanin, which comes in two subtypes, phaeomelanin (red to yellow) and eumelanin (dark brown or black). The mandrill, a primate, is one of the few mammals with blue coloration. The blue color is a structural color, meaning that the molecular structure of the chemicals inside its body produces blue color by scattering blue light. The same is true of blue colored birds, because they use structural color as well. That in turn means that the mandrill lacks melanin in the blue areas of its body, which also means that those areas are not shielded from cancer-causing UV light. Luckily for the mandrill, it lives inside forests, so there is a lot less exposure to harmful UV light. The mandrill also has patches of green colored skin, because all it needs is a small amount of phaeomelanin to produce green color when the yellow pigment is mixed with the scattered blue light.
As other people have pointed out, most mammalian predators have limited color vision. For them, red is indistinguishable from green color. Therefore a reddish brown mammal looks green to them. Hence it is really superfluous for mammals to evolve green color. It is even dangerous to do so, as those that live in open areas will be more likely to suffer sunburn and even cancer when their green colored patches of skin are exposed to sunlight.
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