What highlighting the hair means and how it is done is:
Some of their hair’s colour is removed, to lighten the original colour of the hair, or to change its colour, or to conceal grey hairs or make the hair shinier. It is done by using a powder that is a mixture of magnesium carbonate and ammonia, to which hydroxide is added.
Several methods are used in highlighting the hair, such as rubber caps, foil sheets and rubber cups.
Here we will discuss the most well-known method, which is rubber caps, and we will describe this process which is often asked about. Thus the ruling on it will become clear.
How highlighting with a cap is done:
1. The hair is well combed and combed to the back, taking care to ensure it is dry.
2. The cap is placed on the head to cover all of the hair.
3. Hair is pulled out using a special hook, in the direction opposite to that in which it was combed, taking care to keep the locks of hair similar.
4. The pulled out lock of hair is combed with a fine-tooth comb
5. The highlighting mixture – which is a powder of magnesium carbonate and ammonia, to which hydroxide is added – is applied to each lock of hair that has been pulled out.
6. The lock of hair is covered with a thin plastic cover.
7. The head is exposed to heat to help speed up the lightening process, under constant supervision.
8. When the desired degree of lightness has been reached, the lock of hair is washed with water.
9. The resulting colour is left as it is, or another colour may be added to the lock of hair.
This is the most well-known method. Thus it is clear that highlighting means changing the colour of the hair completely – especially if it is white – and putting another colour, or making the original colour lighter. It is well known that the original colour does not return when this highlighting is done, until the hair grows again. Hence the one who does this process will inevitably lose the original hair colour.
These details have been taken from specialized books on cosmetics.
Several important points should be noted about highlighting which indicate that it should not be used for other reasons. For example: it is not permissible to dye hair black for one whose hair has turned grey, or to resemble kaafir women or immoral women; it is not permissible for a woman to show her hair to non-mahram men; and no harm should be caused to the head by using these chemical substances for highlighting.
If highlighting is free of these concerns, then there is nothing wrong with using it.
And Allaah knows best.