Historical Rules · 1 February 2014
Rule 106.6
Official source text from the 1 February 2014 Rules release
Some spells or abilities that produce mana restrict how that mana can be spent, or have an additional effect that affects the spell or ability that mana is spent on. This doesn’t affect the mana’s type. Example: A player’s mana pool contains {1}{U} which can be spent only to pay cumulative upkeep costs. That player activates Doubling Cube’s ability, which reads "{3}, {T}: Double the amount of each type of mana in your mana pool." The player’s mana pool now has {2}{U}{U} in it, {1}{U} of which can be spent on anything.