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.

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.