dumb edh ?

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ok so say i am playing a few opponents whose generals do not have any black symbols on the card. i have in play an {urborg, tomb of yawgmoth} and a {gauntlet of power}. so my questions is this. if an opponent taps a basic plains they own, can they tap it for the black that {urborg, tomb of yawgmoth} allows, then it becomes colorless and gets doubled with {gauntlet of power}? or regardless of urborg their lands will only tap for colorless and thus wont be doubled by {gauntlet of power}.

[Giggle the Draco Genius]

Hmm that is an interesting question.  As I see it though I would go with
903.9. If mana would be added to a player's mana pool of a color that isn't in the color identity of that player's commander, that amount of colorless mana is added to that player's mana pool instead.

and since gauntlet of power states when it is tapped for the chosen color it would double, since it was tapped for the chosen color but it entered the pool as colorless.

It would double I believe.

Another view to put, is if they are color identity red and they somehow get control of a red/black land.  If the land is tapped it doesn't just automatically make it colorless, it must be tapped for black for it to be converted to colorless.

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found some people discussing it.

http://www.mtgsalvation.com/forums/magic-fundamentals/magic-rulings/magic-rulings-archives/301208-gauntlet-of-power-urborg