Avian predators are not that fascinating and can be killed easily by humans, it requires humans to be always in groups of 3 while moving, with spears, tomahawks, and so on. predators that put evolutionary pressure on humans from hunting havn't been seen, except for the fastest pack animals, wolves, lions, who use mostly scent, in fact all mammals have scent disadvantage.
Animals that hunt in packs, (which would be fascinating) would be a cross between a locusts and bees, and swarming crows. Note that there are no flocks of eagles, because there could be, except that they could empty the entire ecosystem and flocking eagles would be so effective that they would starve themselves.
A equally scientifically likely cause of a color change mutation would be sexual attractiveness, communication, and active hunting ability...
There are different types of metachrosis, spiders even have it sometimes from to adapt to plant colors.
The most cool is the chameleon. It has crystals similar to your nails and hair, contained in soft tissue, which can expand and contract and change density, probably platelets or grains of some kind, and depending on the squeeze, they can reflect light like an LCD crystal or petrol, in fact it's the same as a blue/green/red insect, which all have platelets in their shell which gives a metal reflection.
Chameleons and insects can only do bright colors with protein horn material which is arranged in platelets similar in size to light wavelengths, check TEMicro pics for it... they have expanding melanin organules/cells, black to change them black. images here - https://www.google.fr/search?espv=2&biw=1195&bih=723&tbm=isch&sa=1&q=color+chitine+microscope&oq=color+chitine+microscope&gs_l=img.3...2510.4451.0.4700.13.10.0.0.0.0.332.809.1j1j1j1.4.0....0...1c.1.64.img..10.2.357...0i7i30k1.k9QQZn_J9tc
human eye color changes help humans to indicate the direction of prey and to communicate pathways or dangers without talking, and makes it easier for humans to communicate in groups as it avoids danger within a group. communication seems like a more likey cause for a color change.
Humans with color change would be very communicative about mood and for hunting indications and dangers. long distance communication without waving hands about.
So a human with color change would learn to talk, left right, now, later, etc with colors.
The microbiology is difficult. if humans could do like squids they would have seaslug mucus skin because it would be so flexible and spongey. squids have muscle pores containing red,yellow, black white colors. you could say that they at first developed hair which had iridescent colors, like birds, or even feathers, and learnt to communicate by making color patterns with feathers or by turning their hair like petrol reflections or lcd cells and opals... that's oddly enough one of hte few physical pathways for that trait.
So you haev to figure out a way for humans to show and hide freckles with hair or with other freckles that slide over the first freckles. the most likely way is for humans to develop chameleon skin, so essentially their freckles would become highly developed platelets and humans would have actual muscle or electricl signals going to their skin to make colors by changing platelets or freckles density/postion/rotation.
mammals don't have bright colors, birds do, which is an illustration of the unlikelihood of it happening in mammals, even 60mn years of mouse generations didnt make red/blue/green mice.
Evolutionary pressure by improved communication rather than simple camouglage amd mimickery, and mutation of human pigments to be similar to chameleons, is the mostly likely cause for humans.
because human color blue green red is mostly in the eye, you can say that humans developed colors in their mucus membranes first, eyes, lips, like blue baboons even, and then they developed rgb hairs or even?