Unsure if this should be white or green. My first instinct was white because it's intention is a combat trick. The original cost was , but I thought that was probably too high for a combat trick. Green is more graveyardy than white though. I could potentially even see this in black.

A big p/t boost that benefits from high CMC appears more green than white.

This is also a rather "vanilla" combat trick. Green gets Giant Growth. If this was restricted to a blocking or blocked creature, I would buy into that line of argumentation, but this also is just an optional "win this game by having an unblocked creature" card - that points to both green as well as the higher CMC you have considered earlier.