Another one of those random Google Scholar results:
Control of scrotal colour in the vervet monkey.
J. S. Price, J. L. Burton, S. Shuster and K. Wolff
The vervet monkey has a vivid blue scrotum which pales when the animal falls in social rank. Histology and electron microscopy showed dopa-positive melanocytes in the dermis each packed with fully melanised melanosomes. By transmitted light the scrotal skin was brown on a red background: by reflected light the colour was blue. Thus the blue colour is due to Tyndall scattering over a layer of melanin. Variation of scrotal colour was not due to changes in melanocyte number or dispersion of melanosomes. Pallor was induced by injecting fluid, and blueness could be restored by removing fluid. It is concluded that the blue-to-white colour variation is modulated by the degree of dermal hydration.
I did not know that. A brown-black pigment deep in the skin gives a blue appearance due to light scattering in more superficial skin. How interesting. It's amazing the color variations that you can get from a single pigment molecule, when you throw in various scattering and iridescence effects from tissue structure.
I would never have gotten close enough to find out otherwise.
Oh, give me that disapproving look, will you? Well, would you dissect monkey scrotums?
UPDATE (6/17/2007): A reader e-mails with some updated references on blue skin coloration in mammals, letting me know that it is no longer considered to be Tyndall scattering, but instead color enhancement through phase coherence. Here's a news article from a couple of years ago that describes the research by Richard Prum:
But there is another way to create structural color, and writing in the June 15 issue of the Journal of Experimental Biology, Prum and University of Kansas mathematician Rodolfo Torres showed that the blue in mammal skin comes from "coherent scattering." The best-known examples of this process are the iridescent colors of opals, oil slicks, and soap bubbles. When light hits a layer of a certain material, some of it is immediately reflected by the outermost surface. Another portion of the light enters the material and is reflected back when it hits the bottom surface. If a particular wavelength emerges in phase with its partner in reflection, that color is enhanced; emerging out of phase results in wavelengths cancelling each other out -- and no color.
In skin, the structures that govern this phase shift are collagen fibers about four millionths of an inch in diameter. Because there's no iridescence -- the color doesn't change with the viewing angle -- and because earlier research had suggested that these fibers were arrayed in random fashion, like the molecules in air, the blue-sky explanation seemed reasonable. But using an electron microscope and a kind of mathematics called the Fourier transform, Prum and Torres showed that the collagen in mandrill and marsupial skin actually possessed what Prum dubbed "quasi-order." The fibers aren't packed together in as orderly a fashion as the molecules in a crystal, but they are orderly enough to bring the short wavelengths into phase with each other and create a brilliant blue.
That's even cooler than the light scattering story, I would say.
Now, what have we learned? Two things. First, coherent light reflection can be accomplished by biological structures in a direction-independent way.
Second, monkey scrotum researchers really defend their turf! Or at least, skin pigmentation researchers....
Price JS, Burton JL, Shuster S, Wolff K. 1976. Control of scrotal colour in the vervet monkey. J Med Primatol 5:296-304. PubMed