I heard that the purple color comes from using window cleaner that has ammonia.
I have tint that is about 9 years old and it looks brand new. No color change at all. My previous car had tint and I used ammonia-based window cleaners and it turned purple after a while.
That's what I heard.........plus almost all window cleaners sold in auto stores specifically state that they are ammonia free. Must be something to it.