You mean using the tannin-stained water as the water supply for the system? The RO
should take out the coloration, but once you put it back in the tank it will just get tannin-stained again. If you're just talking about using RO
water during water changes instead of tap, I don't believe that would really have much of an effect. The most effective way to get tannins out IME is putting carbon in your filter, and boiling/soaking the wood as long as possible before adding it to the tank.