Intimidate looks at the current colors of a creature that has it. Normally, Halo Hunter 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.

Halo Hunter's triggered ability isn't optional. When it enters, if you're the only player that controls an Angel, you'll have to target an Angel you control.