Target creature becomes black and gains fear until end of turn. (It can't be blocked except by artifact creatures and/or black creatures.) Draw a card.
Colorless is not a color.
An effect that changes a permanent’s colors overwrites all its old colors unless it specifically says “in addition to its other colors.” For example, after Cerulean Wisps resolves, the affected creature will just be blue. It doesn’t matter what colors it used to be (even if, for example, it used to be blue and black).