My story takes a sort of Shadowrun-esque approach to the concept of fantasy races, in that all members of fantasy races are, or are descended from, former humans transformed by magic, though in this case it was voluntary for the first generation rather than randomly forced.
Basically this is set in a world where humans are entirely normal until their 13th birthday, where their right arm is branded with a swirling pattern of circles that not only allows them to use magical runes, but also makes supernatural changes to their biology, making them stronger faster, tougher, quicker-healing, etc.
But this raises a question: several of the fantasy races have very exotic and often blatantly physically impossible traits, like an extra pair of arms they can materialize from thin air at will, burning up in sunlight, or the ability to levitate and even fly. Why should the children of humans be normal until they turn 13, when the children of fantasy races have their innate racial magic passed down from their parents automatically, brand or no brand?
Which gave me an interesting idea: what if fantasy race children below the age of 13 took on more subdued, mundane, and physically realistic forms, and then on their 13th birthday, when they get their brand, they undergo spontaneous magical puberty and gain all of the bizarre, blatantly supernatural traits their parents have?
Of course, for there to be any point in doing this, I have to have a good idea of what body parts are physically possible, and how much I can make them look like their parents before they start needing magic to function. So I thought I'd start with their most visually obvious difference from normal humans: their skin color.
The fantasy races of my story, rather than ranging from brown to beige, have far more exotic color variations, from cotton candy pink to baby blue to silver and gold. If these skin colors could still be present on pre-magical fantasy race children, it would go a long way to making them visually distinct from humans, even if their other physical features were more limited and subdued. But living things aren't like plastic, and I understand that several colors are actually quite rare in the natural world for various reasons, so I wanted to check this for realism:
By changing only the skin of a creature that is otherwise identical to a human, what natural skin colors besides the human skin range is it biologically possible to have, and which are not?