Convoke (CR 702.51) is a static ability that modifies how you pay a spell's casting cost. When you cast a spell with convoke, you may tap any number of untapped creatures you control as part of paying that cost. Each tapped creature reduces the cost by one mana.
Crucially, the color of the creature matters for colored mana. If you tap a creature that shares a color with the spell's mana cost, that creature can pay for one mana of that color. A colorless creature (or any creature) can still pay for one generic mana regardless of color (CR 702.51b).
For example, suppose you cast Chord of Calling with a cost of {1}{G}{G}{G}. You could tap a green creature to pay for one {G}, another green creature to pay for a second {G}, and a colorless creature (like an artifact creature) to pay for {1}. That would cover most of the cost without spending any mana from your pool.
You can also tap creatures of multiple colors for split costs or hybrid costs, as long as the creature's color satisfies the specific colored requirement being paid. A creature that is both red and green, for instance, can satisfy either a {R} or {G} requirement (CR 702.51b).
Remember that you can only tap creatures you already control (not ones entering as part of the same spell), and they must be untapped at the time you tap them to convoke (CR 702.51a).
Unofficial fan resource — not affiliated with or endorsed by Wizards of the Coast. Answers are AI-generated estimates grounded in the Comprehensive Rules and are not a substitute for an official judge. Verify anything match-critical.