I've always been a fan of the mat black intercooler just for subtlety. Like L18_B110 said it doesn't make a bee's dick of a difference until you start putting thick layers of paint on the core and actually start insulating the compressed air inside it.

As was mentioned earlier, the colour of the surface only has an effect on radiant heat. Radiant heat is a function of some constant of radiation, the surface area and the temperature difference between the surface of the material and ambient temp. As you can imagine the surface of the intercooler will barely rise above ambient making radiation negligible.

I would like to test out a few intercoolers myths, mythbusters style, could be interesting.