How do you stop the color from running on red bath towels?

This is a bit of a quandry. If the towels are new, running them thu the washing machine in a hot water bath with a cup of salt. This "sets" the dyes ued.Then a cold wash will usually finish setting the colors.. If the towels are older and been previously washed, then do the hot water bath with a cup of salt and a cup of baking soda.