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Oracle Text

Creatures you control gain intimidate until end of turn. (Each of those creatures can't be blocked except by artifact creatures and/or creatures that share a color with it.)

Card Rulings

9/22/2011 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.
9/22/2011 A multicolored creature with intimidate can be blocked by any creature that shares a color with it, in addition to artifact creatures. For example, a white-blue creature with intimidate can be blocked by white creatures and/or blue creatures, no matter what other colors they are.