Dye Transfer on Denim?

I have no personal experience with such a problem, but I have a few suggestions.

First, it is the rayon that has caused the problem. Polyester usually is colorfast. Rayon is very tricky. I would wash it by itself in the future; never with anything else.

Try washing the denim dress in hot water and detergent. If the dress is the same quality of denim as jeans, you should n't have to be too fussy with it. Perhaps you shoulld try a laundry detergent that comes with bleach.

You could also try Oxiclean. If you don't care if the denim fades, or if it is pretty light colored already, you could put some bleach in the wash. DONT POUR THE BLEACH on the purpleblotches. Follow the directions on the bleach jug as to how to use it. I have removed dirt and oil stains from jeans by bleaching them. Since faded jeans are attractive (and with work clothes, who cares) the fading, which was very slight, was not an issue.