Mixed Hair Color

Mixing hair color
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Q: Can you mix hair color one day but use it the next? If not, why not?
A: You can NOT mix semi-permanent, demi-permanent, or permanent hair color formulas until you are ready to use them.
Hair colors function as a result of a process in which the hair color (made with aniline derivatives and an alkalizing agent) combine and react with developer (usually hydrogen peroxide) called an “oxidation reaction” which is what causes the formation of the color inside the hair shaft.
We can see the process occurring by watching the color of the combined “color” and “developer” mix grow darker. However, because there is a finite amount of the elements needed to create the reaction to form the color, the “reaction” will wind down once the aniline derivatives, alkalizing agents and peroxide have all combined and reacted. For most color formulas, this means that your color mixture has a lifespan of around 30 minutes from the time it is combined.
The mixture used for bleaching the hair, however, will work as long as the mixture is moist. These off the scalp bleaching powders can be “reactivated” by adding more peroxide and making the mixture moist again. (Although it’s never a good idea to simply let a bleaching mix sit out, and you cannot store a combined bleach mixture in a sealed container.)
This is why I always urge individuals to seek salon professionals before they try to bleach their hair at home. It is too easy to over-process the hair in a bleaching service.
See also:
How to color hair
What does the developer do?
What is the ratio of hair color to cream developer?
Peroxide mixing formula