White is the way to go...but guys i think he asked how to do it ...now to do it for him...
anywyas if your asking that...you simply select the gray things...and you go to image>adjust>replace colour (choose the + eye dropped) select everything...then put the brightness up...
try fidling with the contrast after that aswell..
or u can simply pick the colour of ur car with eyedropped and do it like that..
(sorry was rushing to explain , but u get the point hopefully)