Causes
The first way that hot roots can occur involves two separate color applications. Once permanent hair color is applied and subsequent hair growth occurs, the roots of the affected hair can, and usually do, exhibit a noticeably different color from the ends. To equalize the color of the roots and the ends, another artificial color can be applied to this new growth. This is what is called a "color touch-up." If this touch-up color is lighter and/or warmer than the original color applied, the roots can appear warmer, creating hot roots. This will happen even if the touch-up color is applied from root to end because, as mentioned above, the newly applied artificial color will not lighten the artificial color already present on the ends, as it does the natural color at the roots.
Secondly, hot roots can be the result of a single color process. If a darker shade of natural hair color is being lightened with a permanent hair color, the heat produced by the scalp can cause the hair nearest it to lighten more and/or become warmer than the ends of the hair.
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