Akroan Conscriptor and Leyline Immersion on the battlefield.
Leyline Immersion attached to a legendary creature you control without summoning sickness.
Steps
Activate the creature with Leyline Immersion attached by tapping it, adding ({4} magic symbol) ({U} magic symbol) that can only be spent to cast spells.
Cast Whim of Volrath with buyback by paying ({2} magic symbol) ({U} magic symbol) , targeting Akroan Conscriptor.
Akroan Conscriptor triggers, causing you to untap the creature with Leyline Immersion attached.
Resolve Whim of Volrath, returning it to your hand.
Repeat.
Once you have infinite mana, you may use Akroan Conscriptor's ability to untap any other creature(s) you control.
Results
Infinite colored mana that can only be spent to cast spells.