Is It Best to Have Your Hair Clean or Dirty Before You Dye It?
Hair should be neither too clean nor too dirty before being dyed, so Allure magazine recommends washing it eight to twelve hours before dyeing to get the truest color. Hair that is squeaky clean or exceptionally oily or dirty can interfere with the coloring process when dye is applied.
Most types of dye need to be applied to dry hair, so it's especially important that hair not be washed immediately before dyeing. On a wet or humid day, a hair dryer can be used to ensure hair is completely free of moisture, but give hair about five to ten minutes to cool before applying dye to it.