"Becomes X" without an "in addition to its other [characteristics]" means that the object becomes only X. It loses its other characteristics of the same kind. Cerulean Wisps makes something mono-blue until end of turn.
April 1, 2015 10:59 p.m.
Always check Gatherer before posting a question.
5/1/2008: 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).