Do the official rules of chess forbid promoting a pawn to a piece of the opposite color? Not that it would be everyone's first choice, but it is possible to concoct positions where having an extra piece block your opponent's moves and thus gives you an advantage (think blocking an escape route for a checked king...)
Yes.

FIDE Laws of Chess, rule 3.7e: 


  When a pawn reaches the rank furthest from its starting position it must be exchanged as part of the same move on the same square for a new queen, rook, bishop or knight of the same colour.


The Wikipedia article on promotion contains some information about the history of the rule, which previously implicitly allowed promotion to a piece of the opposite color.