What happens if you leave blue shampoo on too long?
If you leave blue shampoo on for too long (whether you have damaged or healthy hair), you run the risk of leaving a noticeable blue tint on your hair instead of only neutralizing the brassy tones. How long should I leave blue shampoo on? Hairstyling pros recommend sudsing up with blue shampoo once a week to cancel orange or red tones out of brown hair color. For more toning, leave it in your hair for two or three minutes before rinsing.