John, people use everything from Tru-oil to lacquer to help keep case colors from fading. I've tried both and found that Tru-oil did little to preserve them and lacquer had a tendency to wear in odd, blotchy patterns, although it did seem to make the colors last longer. Now I just wipe them occaisionally with a lightly oiled rag and let the colors wear naturally; I like the look, and feel like it's part of having color case hardening.
Joe