Highlights & Dyeing
When mixed with a 10-volume developer, the formula will only deposit color. It will add the color tone to the darker hair and the highlights, but your highlights will still be present, albeit in a different color tone.
If you don't want to alter your highlights at all, but you want to add some more depth to your color, you should consider getting lowlights. Lowlights are applied the same way highlights would be, except that a darker, richer color is used to add more depth to the color. This is best done using foils to make targeting the color more precise. This will allow you to give yourself a darker-looking, richer color overall, without sacrificing your natural highlights.
Foil coloring is best done by a trained professional, and is more expensive than standard color application. The results are well worth the expense. However, if you choose to do your own lowlights, I recommend using a cap and taking care to pull only the darker hair through the cap. The cap method is quicker and easier to perform and can be done by yourself.
The thing to remember with hair color is that as long as the color you are applying is lighter than the natural color, you can retain the lightness of the natural color. For example, if you applied a medium brown color (say color level 4) to your natural hair (say color level 6 with level 9 highlights) you would make both the natural brown and the highlights the medium brown color and would lose the highlights. However, if you used a light strawberry blonde (say color level 9), your natural brown color would take on a warm tone and the highlights would be a strawberry blonde.
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See also:
How to color hair
Highlights Q&A
Foil highlighting
How to apply highlights with a highlighting cap