Skystreamer

Assist allows another player to pay for any amount of generic mana. If an effect changes the cost of the spell, the amount that player may pay will be more or less than the amount in the spell’s reminder text.

Only the generic mana portion of a spell’s cost can be paid with assist. Any colored mana costs must be paid by the spell’s controller, even if the assisting player paid mana of that color.

You may reveal a card with assist and discuss how to pay for it before you begin to cast the spell. Targets are chosen for that spell before you choose another player to help you pay for it and before that player has committed any mana to doing so.