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.

When calculating a spell's total cost, include any alternative costs, additional costs, or anything else that increases or reduces the cost to cast the spell. Convoke applies after the total cost is calculated. Convoke doesn't change a spell's mana cost or mana value.

Because convoke isn't an alternative cost, it can be used in conjunction with alternative costs.

Tapping a multicolored creature using convoke will pay for {1} or one mana of your choice of any of that creature's colors.

You can tap any untapped creature you control to convoke a spell, even one you haven't controlled continuously since the beginning of your most recent turn.

If a creature you control has a mana ability with {T} in the cost, activating that ability while casting a spell with convoke will result in the creature being tapped before you pay the spell's costs. You won't be able to tap it again for convoke. Similarly, if you sacrifice a creature to activate a mana ability while casting a spell with convoke, that creature won't be on the battlefield when you pay the spell's costs, so you won't be able to tap it for convoke.