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I occasionally see people with blue hair and I always assume it is dyed. I wonder how justified is that assumption. Have biologists determined all the possible human hair colors? Or is it possible that if an embryo was exposed to the right amount of radiation in the right place it could get green hair or some other hitherto unseen color?

NCBI lists some alleles of MC1R together with their effects on the phenotype but I don't know how exhaustive their data is (and also other genes might be at play).

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    $\begingroup$ Have you checked Wikipedia? en.wikipedia.org/wiki/Human_hair_color $\endgroup$ Commented Nov 1, 2021 at 3:15
  • $\begingroup$ Do you mean natural? I spent a lot o time in a cancer hospital with my wife and I have seen, for example, pinkish hair that I could bet was not dyed. I've also seen bluish and greenish, but can't begin to guess if it grew like that or was coloured artificially. $\endgroup$
    – Mołot
    Commented Nov 2, 2021 at 9:28

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Melanin is the only biologically coded for pigment amongst mammals and most animals broadly speaking. Some birds are exceptional in this case where reds and greens can be seen (in which case often some dietary component is required for the synthesis of these abnormal pigments).

In humans, the entire variation in skin, hair, and eye color is a consequence of varying treatments of melanin. Melanin traditionally comes in two forms, namely eumelanin and phaeomelanin. The former can come in a brown or black variety while the latter is responsible for the reds and oranges seen in human pigmentation (e.g., localized to the lips, nipples, and nether regions).

Red pheomelanin serves as an accent and does very little as far as protection from UV radiation is concerned; the brown-black melanin serves that purpose. The cells in your body that produce melanin are known as melanocytes, and the MC1R gene you mentioned codes for a receptor that localizes to the membrane of these cells. This receptor acts as a light switch that indicates whether the melanocyte ought to be producing red pheomelanin or brown-black eumelanin. An agonist that binds to this receptor is coded for by the ASIP gene and is also partially responsible for pigmentation differences throughout the animal kingdom.

In most people this receptor functions as intended, and melanocytes in the skin produce varying degrees of brown-black eumelanin (the extent of which depending on one's ethnic background) while pheomelanin is switched on in the few key areas listed above. When both copies of the MC1R gene inherited from each of your parents are disfunctional, this switching mechanism no longer works and your melanocytes will produce primarily pheomelanin ubiquitously across your body. This is what we know as 'redheads'.

What isn't immediately obvious is that red-haired individuals are not unique in just the aspect of their hair. Their whole body presents with a deficiency in eumelanin and as such they also carry a pale/rosy complexion as well as an inability to tan and a propensity to sunburn easily. This is a consequences of the fact that eumelanin is our primary defense against UV radiation.

While melanocytes in the skin and eyes are responsible for the production of melanin, the melanin in one's hair gets there as a result of a handoff between melanocytes and the keratin producing keratinocytes. Melanin within melanocytes is produced and stored within organelles known as melanosomes and, through a complex formed by the 3 genes MYO5A, RAB27A, and MLPH, the transfer of these melanosomes through the tendrils of the melanocytes to the keratinocytes is facilitated. Defects in any of these 3 genes can result in a condition known as Griscelli syndrome (types 1, 2, and 3, respectively) where the transfer of melanin from melanocytes to keratinocytes is impaired.

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As a consequence the hair of one of these individuals appears silver, and when looked at under a microscope you can see the inconsistent nature of the melanin being transferred to the hair during its development:

enter image description here

To summarize, the only pigment that appears in hair is melanin, and thus the only possible hair colors are: the entire spectrum between blond and black; red in the case of MC1R defects; silver in the case of the Griscelli syndromes; and of course white in the case of albinism (which I needn't cover).

enter image description here

You mentioned blue hair color but I think this might be due to a misconception about how blue eyes came to be. Blue eyes are not due to a blue pigment (rarely is blue ever found in nature), but rather due to a physical manipulation of the light being reflected due to the microscopic structure of the eye. So in the same way that the sky and water can appear blue due to manipulation of light, so can the human eye. The blue of human eyes is typically covered up by melanin, except in the case of those carrying a specific one-letter mutation within the HERC2 gene that results in the absence of melanin in the eye.

Not just black and white: pigment pattern development and evolution in vertebrates

Signaling Pathways in Melanogenesis

Rab27a and MyoVa are the primary Mlph interactors regulating melanosome transport in melanocytes

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    $\begingroup$ Bright bird coloring, like butterflies, is usually not the result of pigmentation but microfibers forming a texture that refracts and reflects light almost the same way as a holograph - which explains why the same bird can appear different colors based on the angle they're viewed from. Viewed up close, those feathers are not actually blue or whatever color they appear to be from a distance. $\endgroup$ Commented Nov 1, 2021 at 15:35
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    $\begingroup$ Is it conceivable that human hair could develop (via some mutation) with the microscopic structure needed exhibit other colors via reflection & refraction? $\endgroup$ Commented Nov 1, 2021 at 23:41
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    $\begingroup$ @R..GitHubSTOPHELPINGICE funny you should ask, theres a mutation that causes a condition know as "pili annulati" where the hair follicle has air bubbles in it and they create reflective bands in the hair. I always have my eyes peeled for any new papers about this because the genetic origin of this is not known $\endgroup$ Commented Nov 2, 2021 at 1:56
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    $\begingroup$ In that case, it seems plausible that there may be more colors (depending on your definition of color). $\endgroup$ Commented Nov 2, 2021 at 3:29
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    $\begingroup$ @nick012000 But blue cats aren't really BLUE, at best they're just a sorta-kinda cool toned grey. Another option would be mixed hair like a bluetick hound or a blue roan horse, but this also isn't BLUE, just kinda-sorta-maybe-blueish. You get this with some people who have black hair and are greying from age, but the light conditions have to be very favorable to describe it as blue. $\endgroup$ Commented Nov 3, 2021 at 15:24

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