Black Cars

Can anyone tell me why cars that are painted black are said to get dirty faster? Time and time again people have told me that black paint shows up dirt, mud, wear and tear more than, say, white cars do. Does this really make sense? That is like saying it is easier to see blood stains on a red shirt rather than a blue shirt, or grass stains on green trousers rather than white ones. It would seem to me that the more dirt coloured your car is to begin with, the longer it will be before someone says "Hey, your car sure is dirty." Since black is closer to the colour of dirt than white is, shouldn't it look cleaner? "Hey, is that car dirty?" "No, it is just naturally dirt coloured."