This a proposed task for future genetic engineers.
As you know, when human skin is exposed to ultraviolet light, it generates some protective compounds and changes color. There are two primary compounds, melanin (in the "black" and "brown" branches of humanity) and carotene (in the "yellow" and "red" branches of humanity).
Organic compounds being quite versatile, it is not unreasonable to think that a similarly protective UV-blocker could exist that is white in color. While I'm sure every "white supremist" on Earth would want such a thing as "white melanin" --which actually wouldn't be melanin, of course-- their opinions have nothing whatsoever to do with the rationale for this Idea.
Basically, white substances can reflect other colors of sunlight. So, imagine some "white" person walking across the Sahara Desert, and catching a lot of sunshine in the process. If that person's skin could make a protective pigment against UV that happened to be white, then the skin becomes better at dealing with HEAT, not just UV. Because any color of light that is absorbed instead of reflected becomes heat, after all! And white reflects all colors.-- Vernon, Sep 02 2012 Nanoparticle sunscreen Nanoparticle_20titanium_20sunscreen [bungston, Sep 02 2012] It's not clear that darker skin necessarily causes you to absorb more heat than lighter skin. It would logically seem to be the case, but a quick Google search of the question indicates that the answer may not be so simple. As best I can tell, there's reasonable evidence that dark skin doesn't actually make you hotter than light skin, but nobody is exactly certain why that would be.
This is purely speculation, but I wonder if it may actually have something to do with the heat absorbing/radiating properties of melanin. Since melanin converts UV light to heat, perhaps it's the case that the amount of heat energy given off by this conversion is greater than the energy in the UV light, which would explain why melanin is dark—to absorb the additional heat necessary for this conversion to take place. So with a white “melanin”, you'd actually be losing heat at a substantial rate while in direct sunlight (assuming the “melanin” could absorb enough body heat to maintain the conversion). The net result is that your internal temperature would increase to compensate for the heat loss, actually making you feel hotter.-- ytk, Sep 02 2012 It is possible that both melanin and carotene have a decent amount of reflectance for infrared "colors". Nevertheless, humans have more sweat glands per square centimeter of skin than any other mammal, primarily to be able to dump heat better. And so I continue to think that if we could improve upon melanin (or carotene), to also reflect most ordinary visible light, then that would be a boon to a human body's overall cooling systems.
And remember, colder climates are generally cold because they receive less sunlight. Not to mention people tend to wear more clothes in those places. A white melanin would not be a disadvantage there.
I might mention that one other thing that human skin does with UV light is manufacture Vitamin D. It is known that dark-skinned people have trouble making enough when they move to high-latitude places. Their "default"/normal amount of melanin interferes with production of the vitamin.
Light-skinned humans are adapted to those high-latitude places; they have light skin exactly because of the need to make Vitamin D. The problem these people have is when they go to low-latitude places; their skin can't really make enough melanin or carotene for proper protection. I would hope that any genetic engineers who actually manage to get human skin to make a white UV-blocker could also enhance the ability to make it in quantities appropriate for the latitude!-- Vernon, Sep 02 2012 Some shaky physics tells me that black bodies (in the sense of 'black body radiation') radiate heat better. So a white pigment may not help you stay cooler. Of the people who live in hot, dry climates, I think as many wear black clothes as white.
Also, is it possible to make a white pigment? (Yes, I know titanium dioxide is white, but that's small crystals as opposed to a soluble, biofriendly pigment.)-- MaxwellBuchanan, Sep 02 2012 I wonder if psoriatic skin would be more UV resistant. Lots of creatures in heavy UV environment cope by producing an external layer that is dead / dispensible. One could engineer humans to do this by making their skin hyperkeratotic.
They would not be pretty, these desert dwellers.-- bungston, Sep 02 2012 [MaxwellBuchanan], plenty of natural organic substances are white, and not just because of lack of other colors of pigments. A white flower petal, for example. The white parts of of the skin of a killer whale, for another. And this Idea doesn't ask for perfect white, leaving room for various reasonably effective (more reflective than melanin, at least!) off-white shades.
[bungston], titanium dioxide may be a bad idea, to mix it with organics. Remember that UV activates it such that it promotes chemical reactions with oxygen....-- Vernon, Sep 03 2012 //Also, is it possible to make a white pigment?//
Definitionally, no. But I guess it depends on what exactly you mean by “pigment”. If you're talking about a chemical that dyes some other substance white, the closest I can think of would be bleach (sort of a “negative pigment”, if you will). There probably could be some sort of natural bleach that could be genetically engineered, at least in principal.
//And remember, colder climates are generally cold because they receive less sunlight. Not to mention people tend to wear more clothes in those places. A white melanin would not be a disadvantage there.//
Not much of an advantage either, though. The decreased sensitivity to sunlight on exposed skin might well be offset by the increased propensity towards frostbite.-- ytk, Sep 03 2012 // a chemical that dyes some other substance white, the closest I can think of would be bleach//
Bleach is not a dye, and it works by destroying pigments (which are generally full of vulnerable double bonds).
//Plenty of natural organic substances are white, and not just because of lack of other colors of pigments// I disagree. Teeth are white because they are made of microcrystals of transparent minerals. Flower petals are white because they lack chlorophyll or other strong pigments.-- MaxwellBuchanan, Sep 03 2012 random, halfbakery