If you shine red light at a green plant and green light at a green plant, knowing that a plant must absorb light to grow, which plant will grow faster?

There's no way to tell for sure which plant will grow faster, because
we don't know if a plant gets any benefit from red light. 

But we do know for sure that it won't be the one we shine green light on. 
We know that green light is no use to green plants at all ... that's why
when green light falls on them, they don't even bother absorbing it.
They just let it bounce off, and that's why they look green to us.