It's the most logical doubt. The market is full of purple correctors that mask things for two minutes and disappear when you rinse. "I feel cheated when the effect only lasts a moment". I thought exactly the same. That's why it took me a while to try it.
The difference is PAP+: it's not a pigment that disappears; it's an active, peroxide-free whitener that works on stains progressively, validated in clinical trials with an average improvement of +3.4 shades. The purple colour gives the instant result. PAP+ sustains it.
What finally convinced me was that they're so confident in the results that if you don't see them in 30 days, they refund your money. No questions asked.