Permanent & Semi-Permanent Hair Color

Permanent hair color
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Q: Can a permanent color be applied over a semi-permanent color? If so, how long do I need to wait before doing so? Can a semi-permanent color be applied over a permanent color?
A: Permanent color can always be applied over semi-permanent color, and the time needed to be taken between color applications depends on the condition of the hair in question. Barring any signs of damage to the hair, you could conceivable be safe in applying a permanent color immediately following the application of a semi-permanent one.
You would want to be sure to take into consideration the base colors of the hair colors being used in order to prevent unwanted color results, but generally speaking, applying permanent hair color over semi-permanent color should cause you no problems.
The reverse, however, is not universally true. You can only apply a semi-permanent color over a permanent color if the semi-permanent color is darker than the permanent color. Permanent color formulas may contain hydrogen peroxide developer of a strength sufficient to actually lift the color of the hair to which it is being applied before the color is actually deposited.
Semi-permanent color never provides any lifting action to the hair and therefore cannot be used to lighten the hair whether the hair has been colored with permanent color or not.
See also:
The base color of your hair
The different types of hair coloring
How to color hair
Hair color levels and peroxide developer