On 5/8/10 show, the caller (from Ft. Lauderdale) asked if a black or white car would heat up more. Theoretically, Click & Clack were correct. Also, the Mythbusters have tested this, and found the following (taken from http://mythbustersresults.com/episode38
SPINOFF: A black car heats up faster than an identical white car. (From Pilot 2)
A fan wrote in and asked a follow up question: "Does the color of a car affect the way it heats up?". The MythBusters used two identical cars, one black the other white and left them both out in the summer heat with thermometers in both. By mid-afternoon the black car had heated up to a temperature of 135 ?F while the white car topped off at 126 ?F, almost 10 degrees cooler.