Quoting Dialcaliper:
If you have it suitably shaded (stock bumper), some high temperature "radiator paint" will work fine. If it's hot in the summer where you live, with lots of sun, and you've cut the bumper up, you might run into heating issues, but it hardly makes a difference when air is coming out of the turbo at 150-175 F, compared to the cooling from airflow (painted or not, an intercooler doesn't really do any good when the car is still anyway). Polished aluminum is reasonable for reflecting visible light, but aluminum and copper, polished or not, are actually a pretty lousy thermal surface for heat rejection in the infrared range (polished is actually worse).
That's why most stock radiators are painted black. In reality, you'll probably never notice a difference either way.
If you're still concerning yourself about solar heating, paint it white. White is not quite as good at emitting infrared heat as black, but it is one of the best things for rejecting heat from the sun /ubbthreads/images/graemlins/ooo.gif
Even though airflow cooling is mostly driven by convection, the surface coating still determines the surface temperature, which in turn affects heat transfer to the coolant (in this case, air). So a very hot radiator or intercooler actually rejects more heat than a cooler one, up to the point that the intercooler starts getting hotter than the average intake temperature. That's why radiators usually tend to be quite hot to the touch - if they weren't, they wouldn't be very good radiators.
The larger the difference in temperature between a surface and a coolant, the more heat will be transferred to the coolant. Car radiators (and in turn car engines) would be much more "efficient" (I didn't say powerful by the way) if they were run at much higher temperatures. Unfortunately, the best coolant we have (water/antifreeze) tends to boil and pressurize if you run it too hot. The alternatives are either icky, corrosive, flammable, or some combination (ammonia, various alcohols, steam, etc)
Actually, the whole story is that the process happens both inside and outside of an intercooler (air to fins, fins to outside air), so the most efficient point is when the intercooler is at the halfway point between the intake air and the outside air. If you start changing surface coatings, it changes that efficiency point (to a lower temperature, but efficiency will only improve), but that is the basic theory. Basically, the black intercooler will work better when it is at a cooler temperature, and take longer to heat up. When it reaches the midway point, it will fall back to the same efficiency as the bare aluminum one (unless you also happened to coat or anodize the inside black). A bit of bright sunshine in hot weather might cause you to lose a little efficiency by driving the temperature up, but it's hardly the normal situation.
There are actually very few surface coatings better than black paint for emitting heat, and better than white paint for rejecting solar energy and still emitting heat well. They tend to be rather exotic, very annoying things like "aluminized teflon". For specific situations, there are things like the gold colored "Kapton" foil on the Apollo lunar lander.
Also, at the point that your black intercooler would be "cooked" by the sun, it is probably already very sunny out, and the asphalt is probably approaching 120-130 F anyway, so it's really a moot point. I dare you to put your hand on the shiny aluminum one in that weather...
A very ironic point is that a dirty intercooler has better heat properties than a shiny clean one, provided that the dust and grime are fairly thing, and airflow isn't blocked. /ubbthreads/images/graemlins/rofl.gif
Thank you for the answer, and it was a lot longer than I expected from anyone lol. I think I'm going to go with a radiator flat black. I was thinking about it and if black was so bad why would they have radiators from the factory painted black on a lot of cars?