Melanin -- the molecule -- is ubiquitous (except in arachnids) and it comes in three basic flavors (cf. the link above) called eumelanin, pheomelanin, and neuromelanin. New words for you too?
The most hotly debated type is the first one -- eumelanin; it is produced in 'black' and 'brown' subtypes and is responsible for hair and skin coloration. No less important and ubiquitous -- but usually masked by clothing -- is pheomelanin which is responsible for red hair and freckles, but also the coloration of erogenous zones. Finally, there is neuromelanin which is found in the brain and is accordingly the most mysterious and obscure.*
Eumelanin appears as a polymer in cells. Polymers fascinate me. In a sense, words and sentences are strung-together polymers of letters, as I tried to explain once here. But I digress.
Here's Wiki's depiction of eumelanin -- the type associated with skin color:
The structure screams chromophore but there's something wrong too: the arrow is supposed to represent where the next subunit connects, but if you joined two of these, head-to-head, you'd have exactly two of these units and no more. The normal way to describe polymers to draw the repeating subunits like links in a chain and then discuss how the chains terminate. Oh well.
It's interesting that the subunits in eumelanin and neuromelanin are indole derivatives. That's not to imply indolence -- in the etymological sense -- because higher amount of eumelanin correlates with less sensitivity to sunlight.
Turning now to pheomelanin, a motif of sulfur-bearing chemical morphemes repeats in several directions in pheomelanin:
As a chromophore, pheomeanin is more light selective than eumelanin, reflecting or deflecting reddish wavelengths. This appears to be due to the sulfur and its effect on HOMO-LUMO gaps._______________
*Given the three basic types of melanin and their phenotypical expression, I could have subtitled this post "Race, Sex, and Brains"