Phenolphthalein is an easily available chemical that is an acid/base indicator that turns purple/pink in the presence of bases. It's commonly used in construction to test whether concrete carbonation has happened and for children's toys to make parts change colour magically when they are sprayed with a special solution.
Another fun thing to do with phenolphthalein and some household chemicals is make disappearing ink: some caustic soda, some phenolphthalein, some ammonia and an acid like lemon juice or clear vinegar.
To make disappearing ink you do the following:
1. Take 12g of caustic soda and dilute it in 100ml of water.
2. Next take 10ml 1% phenolphthalein solution (which is typically dissolved in alcohol) and mix with 90ml of water.
3. Take drops of the caustic soda solution and drop them into the phenolphthalein. It will instantly change colour as the indicator reaction happens. Here's the result of a single drop of the caustic soda solution (top) dropped into the phenolphthalein (bottom).
You now have disappearing 'ink' which you can pour onto a piece of cloth. Here's a big splash of that ink on a piece of white cotton:
If you now pour a little ammonia on the cloth the colour will reappear. Another experiment is to take the remaining phenolphthalein solution and add an acid (lemon juice, for example) and see the colour disappear. Add a little caustic soda back and the colour reappears.
PS. If you prefer to make blue 'ink' then use thymolphthalein.