Rule 106.6

Some spells or abilities that produce mana restrict how that mana can be spent, have an additional effect that affects the spell or ability that mana is spent on, or create a delayed triggered ability (see rule 603.7a) that triggers when that mana is spent. This doesn’t affect the mana’s type. Example: A player’s mana pool contains {R}{G} which can be spent only to cast creature spells. That player activates Doubling Cube’s ability, which reads “{3}, {T}: Double the amount of each type of unspent mana you have.” The player’s mana pool now has {R}{R}{G}{G} in it, {R}{G} of which can be spent on anything.

Find this number in the current Rules