As for the white metal discolouring, that depends on the alloy used - some are more prone to yellowing than others. A lot depends on your own body chemistry, too. Does plating come off your rings easily and you have them redipped? Most people know if they tend to wear rhodium plating (or any plating, for that matter) off jewelry quickly or not. Additionally, normally while white gold will lose its crisp, bright, shiny whiteness, even without plating it would normally not be mistaken for yellow gold, especially if it is layered with yellow gold pieces.
Rose gold seems to depend on the person and the brand, IMO. There are rose gold rings that have been rose gold for a hundred years, and then I've seen complaints on the forum about pieces yellowing. I don't think it is as common as white gold discolouring, though.
Like others have said, which you choose depends on your preference, your other jewelry and your skin tone. I commonly wear yellow gold because white just doesn't work as well on me, but there are plenty of people who feel the opposite. I don't wear rose gold at all (just a preference).
