As long as an opponent has 10 or less life, Guul Draz Vampire gets +2/+1 and has intimidate. (It can't be blocked except by artifact creatures and/or creatures that share a color with it.)
10/1/2009 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.
10/1/2009 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.