Cover Gray Hair
A: The general rule of covering gray is that for hair that is less than 10% gray you want to use a color that is approximately two levels lighter than the hair's natural color (or base color).
You can use a temporary rinse to accomplish this, but you have to remember that temporary hair color only lasts until your next shampoo and rests only on the surface of the hair shaft. It can rub off on pillows, clothing and furniture in some cases, so you will want to be aware of potential staining issues.
You might want to consider using a semi-permanent or demi-permanent hair color for this process, since these are not permanent, but last longer, meaning you would have more time between color retouching. These color formulas wash out gradually, usually over 6-12 shampoos.
After you've tested out the idea of covering the gray using deposit-only temporary or semi-/demi-permanent color you can use a permanent color in the same fashion to cover the gray and have more time in between color retouch services. Typically, hair color retouch is done every 6-8 weeks, or when the new growth is sufficient that it becomes visibly apparent.
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See also:
Hair colors and coloring
What is a temporary hair color rinse?
The differences between temporary, semi-permanent and permanent hair color