Colossal Growth

Green combat tricks live and die by whether they only defend or can actually close, and this one splits the difference by making the offensive upgrade optional. Cast it for green mana alone and you get a serviceable +3/+3: enough to win a combat you were losing or push through an extra chunk of damage in a pinch, but strictly a reactive tool. Feed it the red and the axis flips. Trample means a chump blocker no longer eats the whole buff, and haste means the target does not need to have survived a turn to swing, so a fresh creature can attack the moment it lands. That haste clause reveals the real intent: a pure defensive trick has no use for it, while a spell that wants to finish games absolutely does. The Gruul kicker split has a long lineage of pricing an aggressive add-on as a bonus rather than a tax, letting a mono-green deck cast the base version on curve while a two-color deck banks the red for the lethal mode. What keeps both halves fair is that neither leaves anything behind: no permanent stats, no card, just a window. You are paying to make one attack or one block matter more, and the choice of whether to pay the red is the choice of which of those two things the spell is for.