ANSWER:

The layer which prevents the hair from making it wet. Some chemical dye form a layer on the hair and you can peal it and feel how heavy it makes the hair. 

Hair colour usage: Sheik i am confused, what layer actually mean could you please explain it

Beautification & Cosmetic Suregry | October 4, 2018 7:10 am
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I understand that it is not allowed to pluck grey hair and that it they will be light on the Day of Resurrection in sha Allah. Can you please let me know if women can use any colour to dye the grey hair ?