What turns public drinking water purple?

Purple on the fixtures and fittings and plaster = very high chlorine stabilizer or conditioner levels.

It can also be caused by metals and/or minerals in the water, particularly manganese. The manganese reacts to the high level of chlorine (when you shock your pool for example) and turns the water purple. You can use a mineral remover product to clear this up - perhaps Stain Away or something similar to that.