Brassiness is not a thing that happens to you because you did something wrong. It is the default state your hair is returning to, and blonde is the maintenance of holding it somewhere else. Once you think about it that way the whole routine makes more sense.
Toner is a stain, not a dye
Lightening hair exposes warm pigment underneath — orange on darker hair, yellow on lighter. Toner deposits a cool tone over the top to cancel it out. It sits on the surface, it does not penetrate, and it washes out. Every time. There is no toner that doesn't fade and no salon that can prevent it.
So a blonde going warm after six weeks is not a failed appointment. That's the expected lifespan, and the plan is to slow it down rather than to stop it.
Purple shampoo, once a week, no more
Purple shampoo is a toner. Using it every wash is over-toning, and over-toned blonde goes dull, slightly grey and lifeless — the exact opposite of what people are chasing. Once a week, left on for the time on the bottle rather than an arbitrary long soak, is enough for most people.
If your hair is already looking flat rather than brassy, stop entirely for a fortnight. It comes back.
Heat, sun, chlorine, and the invisible one
Heat opens the cuticle and toner escapes faster, so cooler water and a heat protectant both buy you time. Sun and chlorine oxidise the pigment directly. None of that is surprising.
The one that catches people out is hard water. Mineral deposits build up on the hair shaft and pull blonde orange, and it happens fast enough that people assume their colour was done badly. If you moved house and your blonde changed character within a month, that is almost certainly why. A shower filter is a genuinely unglamorous purchase that solves it.
Toner fades, purple shampoo weekly, cool water, and check your water before you blame your colourist.