Give this a read if you want to know why. I've also copied it below.
http://www.offroaders.com/tech/paint-it-black.htm
It's been said that black is actually the best color for dissipating heat from the engine. This statement is true, here's why:
The reason black paint dissipates heat more than any other color is that black is the most capably dissipative color for infra-red (heat) wavelengths. As we all know, back is very absorbent when energy rays (visible AND invisible) such as sunlight hit it. It is also able to cast off the most heat energy, all of its radiation being in the infra-red part of the energy wavelength spectrum. Single colors tend to focus their ability to both absorb AND dissipate in the wavelength of their color, and far less in the infra-red compared to black. One last thing about black: It absorbs energy better than it dissipates it, which is likely why the effect of black dissipating heat is less known. Also, since it absorbs far better than it dissipates (as do all colors), it has a net gain, for example when your black car sits in the sun, until an equilibrium is reached depending on the heated body's ability to be cooled (like with a fender or hood, by the surrounding air). The ambient temperature of the air around the black fender keeps it from heating beyond a certain range. I hope this helps explain the "I don't know why" of black paint being a better cooler for engines. The effect won't be much, and might not even be noticeable unless closely monitored, but it is a fact.
Black radiates heat in the infra-red spectrum better than other colors, which tend to "specialize" by radiating less efficiently in the infra-red and more in just their color portion of the spectrum.
Speakers analogy: With regard to black being able to radiate heat well, heat is like a bass note, black is a woofer, and other colors are "mere" tweeters.
