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A purple color-correcting shampoo will counteract the yellow tones and make your hair appear brighter and blonder. This is not as strong as toner, since it is intended for regular use. Here are a few examples of purple or violet shampoos:  Tressa Watercolors Violet Washe Shampoo John Frieda Sheer Blonde Color Renew Tone-Correcting Clairol Shimmer Lights Pravana Pure Light Brightening Shampoo for Blonde, Silver, or Highlighted Hair Matrix Total Results Brass Off shampoo Paul Mitchell Platinum Blonde shampoo Joico Color Balance shampoo Any shampoo with "D&C Violet" or "Extract Violet" in the ingredients list Follow the directions on your shampoo bottle. Apply the purple shampoo instead of normal shampoo, then rinse out and follow it with conditioner. Most purple shampoos need to be left in for 3-5 minutes for best results. Too much purple shampoo will leave your hair with a light purple tint. Apply every two or three washes, or even less often once the brassy tones have gone.
Select a purple shampoo for regular upkeep. Wash your hair with purple shampoo. Repeat every two or three washes.