If you like the color of box elder, a clear lacquer or poly would be best. I don't like the water-clear lacquers and polys. They seem to impart a blue cast to the wood -- like a bad florescent light.
As mentioned, the flame color will eventually turn brown. Keeping the wood out of sun light will delay the browning, but it will happen eventually. I have a lovely piece another turner made for me out of flame box elder. He airbrushed the flame with color-fast pigment. I'm sure the "real" flame color has gone brown, but you cannot tell because of the pigment he added.