Upon needs thresholds being crossed human bodies would change instantly to colours etc etc to help out.
They did change. its reasonable God knew we needed to change and so gave us the ability to change. Like creatures.
Then its obvious colour etc change had to be instant. Any intermediate stages would not have a need to change more.
You must allow our glorious genetic beings to be able to change in these minor ways.
Like those sea creatures that change instantly. No big deal in nature.
These mutations, I suggest, are only special cases of mutations gone wild. Mutations are possible a natural normal part of biology. Or rather not really mutations. Speculation but better then evolutionism.