When I Google Image search "wizard" (http://imgur.com/ssldJUy), most of the figures are dressed in blue or purple robes, often with stars, and usually with a matching pointy hat. Have wizards always been depicted this way? How did this tradition get started?
I'm going to go with Gandalf specifically here, since he's more or less the archetypal wizard that sits in the modern psyche.
Tolkien, in Letters 107, described Gandalf as a "Odinic Wanderer." The similarities between Gandalf and Odin in his portrayal of the Wanderer are pointed out by several other people, including "Tolkien's Ring" by David Day.
Odin was described as going amongst the people with a wide-brimmed hat and a robe, dispensing knowledge and playing "tricks" on people. The similarities are definitely there.