Salon and Home Colors Difference

Home hair color
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Q: I used to have bleach highlighted hair for 10 years, then 1,5 years ago I fancied change and had my hair colored honey brown at the salon (full color + high-lighted blonder tones). But now I would like to go back to blonde highlights, and but instead going back to the salon, I purchased home highlight kit from a local chemist.
I did the skin sensitivity test on my elbow and got an itchy-feeling reaction, like a nettle sting. I was surprised, as I thought I wasn't allergic to hair colors. (My hair has been colored in a salon for years.)
My question is: is there a difference between salon colors and home colors?
My own hair color is dark brown. Among the full head color I had done and maintained in a salon were blonder tones and I thought they were bleach ones. Is there a difference between bleach highlights and other milder highlights? Does this test mean I have turned sensitive to highlights? Should I go back to a salon for another skin test?

A: As to the question of whether there is a difference between salon colors and hair colors designed for at-home use the answer is "yes." Many times the at-home color formulas contain added ingredients to help combat common problems encountered with hair color processes.
They will often contain ingredients to soften the hair and provide deeper penetration of the color, or to add extra conditioning to the hair. Salon color formulas tend to be stronger and in some cases can only be purchased by licensed professionals who have been trained to handle the color formulas.
It is entirely possible that you have developed sensitivity to a common ingredient of all hair colors or that you were simply sensitive to an ingredient found only in the brand of hair color you used at home.
Regardless of which, you should be sure to tell your stylist about this reaction when you next visit the salon and make sure he/she performs a preliminary test before another color service is performed on you. This will help you to prevent an unfortunate reaction.
As to the question of the difference between bleach highlights and other milder highlights, the difference is in the strength of the chemicals used. For bleached highlights the hair is treated with a bleaching agent catalyzed by crème developer. This mixture will continue lightening the hair as long as the mixture remains moist and can result in over-lightening and burning the hair if not closely monitored.
Other lightening formulas use a high-lift developer with a toning color and generally have a "life" of about 30 minutes from the point of combination, after which the formula becomes inert and no longer works.
These milder formulas are great for adding highlights as much as 3-4 shades lighter (possibly 5-6 shades if heat processing is used) and of a certain tone of color, but will need to be carefully monitored when using the stronger forms of developer (such as 40-volume peroxide and higher).
See also:
Hair coloring
Hair color allergies
Burning sensation on the scalp when coloring hair