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.
Only the generic mana portion of a spellâ€™s cost can be paid with assist. Any colored mana costs must be paid by the spellâ€™s controller, even if the assisting player paid mana of that color.
If an effect allows a player to cast a spell spending mana â€œas though it were mana of any colorâ€ or â€œof any type,â€ that player must still pay for the colored mana in that spellâ€™s total cost. That cost doesnâ€™t become generic.
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 spellâ€™s reminder text.