You can promote up to one level under you. However this is done only under the most dire of circumstances. For example, a guy in Guam who has no black belt readily availible really wants to promote this blue belt to purple so he can compete in the Pan Ams, but can't wait for the black belts visit in 9 months.
It would be HIGHLY inappropriate, for example, for me to promote a whitebelt at my gym. My instructor would likely beat me unconscious. And who would you rather get your blue belt from? Some random scrub purple belt or a 4th degree Carlson black belt? No brainer.