Tome of the Guildpact

The trick is that the fixing and the payoff live in the same slot: five generic mana buys a source that produces any color and refunds a card every time you cast something multicolored, so the more gold your list runs, the more the second line pays out. Most any-color mana producers ask you to build a heavily multicolored deck and hand you nothing but the mana in return; this one ties the fixing to a reward, which sharpens a real tension. Both abilities want the same board state, and half-measures get punished. A lightly gold deck runs a slow producer that occasionally cantrips; go all the way and it becomes a genuine engine, tapping to cast the very spells that then draw you cards. The trigger fires on the cast, not on resolution, so a countered or fizzled multicolored spell still cantrips, and "multicolored" checks the spell's actual colors on the stack (two or more colors from mana cost, color indicator, or a characteristic-defining ability), not Commander color identity: a gold card whose cost is not fully paid in every color still counts as long as it is genuinely two-or-more colored. It is a purpose-built reward for the gold-matters archetype, priced high enough that you do not slot it just to smooth a manabase. You run it because your list already drowns in multicolored cards and wants to be paid for it.