Hall of Gemstone

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.

World Championship Decks 1998

Mirage

During each player's upkeep, that player chooses a color.
Until end of turn, each mana-producing land produces mana of the chosen color instead of its normal color.

Although the color may be altered, any restrictions on the use of the mana are not removed.

Will not affect lands that do not produce mana.

This has the supertype world. When a world permanent enters, 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 at the same time, they're both put into their owners' graveyards.

Any colorless mana that's produced isn't affected.