Wash the red material a couple of times before you make the skirts to rinse any excess dye out.

Vinegar doesn't work with modern cotton dyes, nor does salt. And pre-washing is no guarantee that any bleeding will not continue with subsequent launderings.

I'd test the reds first to see if they run. Wash them individually in a sink and see if the water turns pink. If it does, you can treat the offending fabrics with a product called Retayne. I've had really good results with it. Google it to find a source.