How is this different from purple shampoo or permanent hair dye?

Purple (or blue) shampoo contains direct dyes designed to counteract unwanted yellow or orange tones — it is a colour corrector, not a colour depositor for a specific blonde shade. It won't make medium-brown hair look lighter or add a new blonde tone; it only cools existing blonde or lightened hair. Permanent oxidative dye uses ammonia (or an alternative) and peroxide to open the cuticle, remove natural melanin, and lock new colour molecules inside the cortex — the result is permanent until the hair grows out or is cut. This toning shampoo sits between the two: it deposits cosmetic pigment on the outside of the hair shaft while you wash, giving a visible but temporary tone shift (e.g. a fresher, lighter-looking blonde) without altering the hair's internal structure. It also cleanses in the same step. Choose purple shampoo if your only goal is to remove brassiness; choose permanent dye if you need lasting grey coverage or a permanent shade change; choose this product when you want a short-term refresh, a subtle blonde boost, or to test a lighter direction before committing.