The colorless mana symbol {C} is used to represent one colorless mana, and also to represent a cost that can be paid only with one colorless mana.
May 1, 2014 · Journey into Nyx
Numeral symbols (such as {1}) and variable symbols (such as {X}) can also represent colorless mana if they appear in the effect of a spell or ability that reads “add [mana symbol] to your mana pool” or something similar. (See rule 107.3e.)
September 30, 2011 · Innistrad
Numeral symbols (such as {1}) and variable symbols (such as {X}) can also represent colorless mana if they appear in the effect of a spell or ability that reads "add [mana symbol] to your mana pool" or something similar. (See rule 107.3e.)
June 17, 2011 · Commander 2011
Numeral symbols (such as {1}) and variable symbols (such as {X}) can also represent
April 1, 2011 · Duel Decks: Knights vs. Dragons
Numeral symbols (such as {1}) and variable symbols (such as {X}) can also represent colorless mana if they appear in the effect of a spell or ability that reads "add [mana symbol] to your mana pool" or something similar. (See rule 107.3e.)
February 4, 2011 · Mirrodin Besieged
Numeral symbols (such as {1}) and variable symbols (such as {X}) can also represent colorless mana if they appear in the effect of a spell or ability that reads “add [mana symbol] to your mana pool” or something similar. (See rule 107.3e.)
October 1, 2010 · Scars of Mirrodin
Numeral symbols (such as {1}) and variable symbols (such as {X}) can also represent
February 1, 2010 · Worldwake
Numeral symbols (such as {1}) and variable symbols (such as {X}) can also represent colorless mana if they appear in the effect of a spell or ability that reads "add [mana symbol] to your mana pool" or something similar. (See rule 107.3e.)