Diablo 3 Rainbow Portrait is a vanity item in Diablo 3. It is obtained by killing Sir William in Whimsydale. He has a very low chance of spawning, but if he appears, he is guaranteed to drop a book that will give player a Whimsyshire-themed portrait.
Staff of Herding is required. If you do not have it, you may add it for an extra $15.
A delay may occur in some cases such as but not limited to: start of a new season, tight or busy schedule and public holidays.