I'm making a genus of elves, native to an African-like continent and I want them to have green hair that they put together to make grass like dreadlocks.
Could they pull this off or are they going to have to grow gardens on their head?
Human hair color is based on two pigments: eumelanin and pheomelanin. The more eumelanin you have, the darker brown-to-black you get. The more pheomelanin you have, the more orange-to-red you get. Add the two together and you get auburn. So, based on "reality," there is no way to get green hair from brown, orange, and red.
But, you're dealing with elves. What's reality to an elf? Isn't life a dream with humans starring as the nightmares? Youbetcha! So, let's use reality to create a sensible-sounding reason why your elves have green hair. Let's suggest that their genome has a third pigment: jordalanin. Jordalanin creates green.
If Elves have all three pigments, it means they can be blond, brunette, ginger, green, and the mixing derivatives that would include a true gold and even a dark blue (a real blue, but only with all three pigments, methinks). Use a graphics tool to mix color between brown, red/orange, and green and see what you can come up with.
With just two pigments, you'd get:
Jordalanin + eumelanin = brown, black, and green with subcolors including emerald and dark forest green.
Jordalanin + pheomelanin = red, orange, and green with subcolors including gold and copper (in a way humans can't. Human copper hair is really just a dark red. Real copper has a yellow component humans can't create with just two pigments).
You could have a lot of fun with this!
Polar bears can get green fur.
Polar bear hairs are hollow and clear. When circumstances are right, algae find the inside of the hair to be a nice place to live.
from The greening of polar bears in zoos. R A Lewin P T Robinson Nature , 1979, Vol.278(5703), p.445-447
We first supposed that the colour was due to green algae such as Chlorella or Scenedesmus on the surfaces of hairs, growth of such algae being promoted by the presence of nitrogenous wastes in the waters of the bears' pool. (The pool in the exhibit area, which contains 12,500 gallons of tap water, is drained and cleaned twice weekly.) However, microscopic examination of samples of hair taken from the three San Diego bears and from a similarly green polar bear in the zoo at Fresno, California, revealed that this was not so. The outer surfaces of the hairs appeared clean and smooth, except for the normal squamation. The coloration was clearly attributable to the presence of algae inside the hairs, specifically in the hollow medullae of many of the wider (50–200 µm), stiffer guard hairs of the outer coat. (The thinner (<20 µm) and more undulant hairs of the under coat, which were not hollow, were colourless.) Some of the lumina were apparently filled with air, but many of these hollow spaces were partly occupied by masses of small greenish cells, which we describe here.
So your green haired creatures could go about this the same way: clear, hollow hairs within which grow algae.