I am going to make an extract from red cabbage or purple cabbage or red kraut with Isopropyl alcohol or rubbing alcohol. This purple tincture indicates PH turning to blue and green in basic (or alkaline) medium and into pink and red en acidic medium. In case you don't have isopropyl alcohol, you can do it with ethyl alcohol (ethanol). This happens because of red cabbage contains some substances called anthocyanines, present in many vegetables.
https://en.wikipedia.org/wiki/Red_cabbage
https://en.wikipedia.org/wiki/Anthocyanin
https://en.wikipedia.org/wiki/Rubbing_alcohol
https://en.wikipedia.org/wiki/Isopropyl_alcohol
https://en.wikipedia.org/wiki/Tincture
https://en.wikipedia.org/wiki/PH