If tapping a creature using convoke wouldn't pay for any mana in that spell's cost (because you've already paid for all of the mana in the spell's cost by tapping other creatures or because you've already paid for all of the generic mana in the cost by tapping other creatures and the creature you want to tap doesn't share a color with any of the remaining mana symbols in the spell's cost), you can't use convoke to tap that creature. Effects that increase or reduce the cost of a spell with convoke apply before costs are paid and will increase or decrease the total amount of creatures you can tap.