Some of the responses are pretty funny. ::008:: Yes, black helmets are warmer than white or light colored helmets. To a degree, the importance of this depends on your riding. When I rode everywhere and only had a bike, black helmets sucked, because I commuted on the bike too, so had daily traffic to be stuck in. Even in Oregon, at 60F, sitting in the sun in traffic was a lesson in how to bake your noggin in a black full face helmet. White, not big deal. OTOH, if you don't commute and ride mostly where you're not stuck in traffic or in the desert, go for what ever color floats your boat. Ditto if you're talking about a half shell helmet, it's not as noticeable.
I know lots of guys that buy color matched helmets for their bikes. Good for them, no fault there. Not my thing. Seems like the more bikes I've had and the more miles I've ridden, the less something like helmet color matters. I buy white because plain colors are cheaper, I live in a world where moto cops often where white helmets and people are people. Living in the desert it makes sense too.
Do what makes you happy. (but matte color helmets do look dorky, imho)