Generally, a purple shampoo is left on wet hair for 3-5 minutes or, as stated in the ‘directions to use’ on the product.
It would be best to try it out and find the specific time your hair needs to tone down the brassiness.
When I started using purple shampoo, I kept it for 2-3 minutes as indicated on the bottle, but later, I realized leaving it for 5-7 minutes worked for me.
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