I checked around the University of Victoria chemistry department, and most people suggested phenolphthalein - however that is really more pink than violet. One person suggested copper sulphate solution, which is pale blue but turns a deep violet blue with ammonia. The Merck Index in its Miscellaneous Tables section has a huge table of indicators. Here are some more suggestions:
- Methyl violet is yellow in acid and violet in base (ammonia)
- Bromophenol blue is same
- alizarin sodium sulphonate is same
- quinoline blue is colourless in acid/neutral and violet in base
- m-cresol purple is yellow in neutral/acid purple in base
- cresolphthalein is colourless in neutral/acid purple in base as opposed to phenolphthalein which we mentioned is pink in base
The principle is that the ammonia creates a basic (i.e. not acidic) environment and therefore the indicator changes colour. Good luck!