G.4.1- Colorless mana is mana which has no color. Some spells and abilities may generate this kind of mana. [D'Angelo 1998/02/03]
G.4.2- Colorless mana is specified in text such as "one colorless mana". [D'Angelo 2002/02/01] Colorless mana may also be specified on cards using a gray circle with a number in it. [WotC Rules Team 2002/02/01]
Note- Also see Generic Mana (Rule G.17).