Enchantment - Aura, 1G (2) Enchant land Whenever enchanted land is tapped for mana, its controller adds one mana of any color to his or her mana pool.1
Illus. Heather Hudson
The color of mana to be generated is chosen when tapping the land for mana. [Urza's FAQ 1998/10/05]
This does not change the mana type the land produces for any other effects that check what mana type the land produces. For example, a Mana Flare would see a Mountain produce {R}, and make an extra {R} regardless of what color Fertile Ground generates. [Urza's FAQ 1998/10/05]
Still generates mana even if the other land is tapped for zero mana. For example, Gaea's Cradle tapped when no creatures are in play. [Invasion FAQ 2000/10/03]