heat exchangers on production cars are painted as the car manufacturer wants them to last more than 5 mins after the roads have been salted

whether they are painted silver or black is largely down to manufacturer preference, i.e a silver one is easily visible through the front grille, but a black one is hidden.

if anyone (e.g. proalloy or whoever, or a magazine) wants to do a proper scientific test, the test facility that i work on could be used. 250 bhp rolling road in a temperature and humidity controlled test cell with full vehicle frontal area windspeed up to 70mph.