Since the stones are made from industrial diamonds there is a good chance that the red is not really worn out, but just clogged up with base material. I have found that I need to clean the stones occasionally with mineral spirits or the diamonds (especially the small ones on the red stone) will become contaminated and just slide over the edges.
As for use, I think the red ones seem to be overkill unless you are a World Cup racer. I use the blue ones on a regular basis and don't feel that my edges need to be any smoother. If you want a shaper edge you can increase the bevel to less than a 90 degree angle. For ski trips I'll just take the black one and use it for temporarily taking care of any bad edge hits.