Clarifying shampoo- aka purifying, detox and deep-cleansing shampoo.
Even your standard shampoo and conditioner can build up their own residue over time.
If you use styling products, dry shampoo, or spend time in chlorinated pools, then use clarifying shampoo once per two weeks. If you have dry coloured hair, limit it to once a month, since clarifying formulas can strip the colour.
Without a doubt, use it before you are going to the hair salon to colour your hair.