At the beginning of each player's upkeep, that player chooses a color. Until end of turn, lands tapped for mana produce mana of the chosen color instead of any other color.
10/4/2004 Although the color may be altered, any restrictions on the use of the mana are not removed.
10/4/2004 Will not affect lands that do not produce mana.
10/1/2008 This has the supertype world. When a world permanent enters the battlefield, any world permanents that were already on the battlefield are put into their owners' graveyards. This is a state-based action called the "world rule." The new world permanent stays on the battlefield. If two world permanents enter the battlefield at the same time, they're both put into their owners' graveyards.