from Wiktionary, Creative Commons Attribution/Share-Alike License
- n. A pH indicator that can also be used to make invisible ink.
Sorry, no etymologies found.
I did find instructions for making colored bubbles, but it involved stuff like sodium hydroxide and blue thymolphthalein and red phenolphthalein.
In these cases, phenolphthalein, thymolphthalein or other compounds that are colorless in acidic ranges are often used.