Heat Exchanger Paint
The tube portion of the IC should be painted with special radiator paint. Regular enamel will go on too thick, and cut down on the cooling. Paint is not a very good conductor of heat. I'm not sure what the radiator paint is, but it goes on thin, and radiator shops should have it available. The temperature of the inlet/outlet boxes should be low enough for regular engine paint, in whatever color you like. The paint on mine appears to be a semi-gloss, probably baked on at the IC factory. It's pretty durable.