I forget, do you use co2 or ferts in your tank?
IMO, water changes too often may stop them from breeding, however it won't kill them provided you keep parameters correct You'll receive different answers from different folks, and that's what makes shrimping interesting.
When I kept my cherries too low in gh, I found dying off, but when I raised it to 6+ they did well.
Purple can be caused by too much co2, old age, blue tissue under the shell, astaxanthin, death, and a variety of other issues. Unless a majority of your shrimp were turning purplish, I wouldn't consider that a symptom.
As an emergency measure, have you considered adding a cup of ammocarb inside a tied off stocking and placing it behind your bubbler?