One card (Drain Power) causes one player to lose unspent mana and another to add "the mana lost this way." (Note that these may be the same player.) This empties the former player's mana pool and causes the mana emptied this way to be put into the latter player's mana pool. Which permanents, spells, and/or abilities produced that mana are unchanged, as are any restrictions or additional effects associated with any of that mana.
Drain Power and Ancient Ziggurat. Nicole controls an Island and an Ancient Ziggurat. Ana casts Drain Power targetin Nicole. Nicole taps the Island for a blue mana, and the Ziggurat for a green mana. After Drain Power resolves, Ana my use the green mana only to cast creature spells.