حماية لون الشعر

Less than you wash uncoloured hair. Every two to three days if your scalp allows, and use dry shampoo in between rather than pushing through a wash you don't need.

For fresh colour, yes, it's a sensible precaution — sulfates clean more thoroughly and take more pigment with them. It's less important once colour has settled.

Usually water quality, sun, or the underlying warm pigment showing as the toner washes out. Toner is semi-permanent by design; the brass underneath was always there.

Only if you have lightened pieces — highlights or balayage — that are going brassy. On unlightened dark hair it does nothing.

Indirectly, yes. Better-repaired hair has a tighter cuticle, and a tighter cuticle holds pigment. It's not a colour product but it helps.