It's been a while since I've dealt with this, but isn't white smoke an indication that gas is not burning off correctly. In other words, the engine is running to rich with gas.
Black smoke is an indicator that motor oil is leaking into the engine and burning, causing the smoke to be black.
There is another possibility, on vehicles with automatic transmissions, if the transmission fluid is dripping on the hot exhaust pipe, the transmission fluid will vaporize and turn into white smoke. Although, that won't be coming from your exhaust pipe and when that happens, it looks like the entire vehicle is on fire. Happened to me once, scared the bee-jee-beers out of me!
Well, that's my input for the day. :)