Lighter Hair Color
A: There are a couple of factors that come into play here. People usually apply the color to the top part of the head first as this is the most accessible part. This means that the color stays on a longer time period on this part of the head than under or on the sides, resulting in a lighter color than the rest.
When you dry your hair with the hair dryer, you also tend to concentrate on the top part more, because this is the most visible and accessible part of your head. Thus the top part is subject to more heat. When you flat iron or curl your hair with tongs, you also tend to concentrate more on the top part, because this part is styled easier as you can see and reach it easier. All this heat-processing lightens the top hair considerably.
Thus, when you apply color to the lighter hair on the top half on your head, it is understandable that the color will be lighter on this part than that of the darker hair lower on the head. You will see that even people who don't color their hair at all tend to have lighter hair on top than at the bottom due to sun exposure and other factors.
Also, most styles are colored this way as well as it looks more natural. A lot of blondes will have the top part of their head highlighted, while having the bottom half colored in a medium or light brown. Dark heads are also prone to follow the same technique, getting the top part of their head colored a deep red or a lighter brown, with the bottom part dark brown or even black.
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See also:
Hair coloring
Advanced hair color tips
What can go wrong with hair color services