Animals like birds have a spectrum of colors available to them, but less so can be said for mammals. There are definitely some really striking patterns in mammals, but that tends to consist of black and white patterns - like a skunk's stripes or a ring-tailed lemur's tail.
But one of my favorite animals is the mandrill, a colorful large monkey that has distinct blue and red on its face. The red skin is not too uncommon, and can be caused by vascularization (blood flow) close to the surface. What about the blue?
It's kind of strange. Sometimes animals display color structurally, not with pigmentation, where the microscopic physical shapes of the region reflects light in a way that makes the color. This is true for some birds. It's due to the arrangement of protein fibers in the skin or feathers and how they scatter light. Apparently there's more than one way of doing this, and different species have come to the same conclusion (blue color) via different means (different ways of scattering light).
Bluejays use microscopic bead designs in the feathers. When the beads scatter light, only the blue light can escape. Peacocks use a different method of microscopic lamellae.
Whatever structure the animal uses, it relies on microscopic ridges and branches that are parallel to each other. When light hits the branches, some light reflects off of the structure (reflection #1) and some light enters the branch, reflecting off the bottom layer of the branch (reflection #2).
For most colors, the two pieces of reflected light are called 'out of phase' - the crests and troughs of the light wave do not line up (remember light is technically a wave). So we can't see these.
For the color blue, the two pieces of reflected light are in phase and do not cancel each other out. We can see the blue color.
Mandrills rely on this structural coloring, the same way that blue butterflies do. Butterflies built this light-scattering structure out of the scales on their wings, and mandrills built it out of their skin cells. Apparently, blue or green eyes in humans is also due to color scattering, and not pigment. There is no blue or green pigment in the human iris.
There are three other mammals that also have blue coloration, and they achieved it in the same way - structural coloring. These are the vervet monkey and two marsupials: the mouse opossum and the wooly opossum. The blue only appears in males of these species because it only shows up on the testicles during the breeding season (not including photos, sorry!).
lucky instar 