Intimidate looks at the current colors of a creature that has it. Normally, if Guul Draz Vampire has intimidate, it can't be blocked except by artifact creatures and/or black creatures. If it's turned white, then it can't be blocked except by artifact creatures and/or white creatures.
If an attacking creature has intimidate, what colors it is matters only as the defending player declares blockers. Once it's blocked, changing its colors won't change that.