Other answers are spot on. I will point out that many plant-based dyes are actually acid-base indicators. Check the Internet. Also, pH meters are. show more Other answers are spot on.
I will point out that many plant-based dyes are actually acid-base indicators. Check the Internet. Also, pH meters are available from electronic component suppliers. If you buy one of these, be sure to get the necessary buffer solutions for calibrating it and always store it with the electrode in a strong solution, never plain demineralised water (but rinse it in demin. between tests to avoid contaminating your samples).
Any acid or alkali which has a different colour than the ion it forms will change colour depending on pH. If your indicator is HInd and dissociates into H+and Ind-, and Ind- has a different colour than HInd, then adding
it to an acid (where there are already plenty of H+ ions present) will tend to keep it from dissociating and so the solution will keep the colour of HInd. Adding it to an alkali will cause H+ ions to react with OH- and so dissociate more HInd molecules, changing the colour to Ind-. Similarly for an indicator of formula IndOH which dissociates to Ind+ and OH-.
Homemade plant dye indicators should ideally be calibrated against a known-working pH meter (or freshly-prepared buffer solutions) as soon as possible before use, and stored airtight and out of strong light. If you intend to make heavy use of them, it is worth performing your own experiments to determine if and how they degrade; even if only for the benefit of other guerilla chemists and those who rely upon their services :)