Most of the time I don't use stop bath at all, just a brief dunk in water to get rid of most of the developer carry-over. When I do use stop (mostly for lith-printing when seconds really matter), I make new every time:
Go to a supermarket. Buy Citric acid. Dump about 8 grams (about one third of a 25-gram packet) of this in about one liter of water. Use until finished developing that day. Then dump the stop, and mix new the next time. No indicator needed. The capacity of this stop is far greater than acetic acid, the pH is higher (less risk of outgassing from carbonate in the devloper), and the smell is a lot nicer.
Hans (clogz) is probably right about the indicator used. Bromochreosol Purple (5',5"-Dibromo-o-creosolsulfone-phthalein) changes colour from yellow to purple in the pH range between 5.2 and 6.8. You can use it if you must, but I see no reason for it.