Is Dying Hair Prohibited in Islam?
Question

Is it permissible to dye the hair with modern dyes such as the products of L’Oreal , Bigen and other companies?
Praise be to Allah.Using dye on the hair comes under the heading of customs, and the basic principle is that it is permissible and allowed.
Based on that, it is permissible to dye the hair with modern dyes and other substances, so long as the dye is not black that is used to cover grey hair, and does not involve imitating the disbelievers, and is not proven to be medically harmful.
It says in Fatawa Nur ‘Ala Ad-Darb by Shaykh Ibn ‘Uthaymin (may Allah have mercy on him):
“The basic principle with regard to matters other than acts of worship is that they are permissible. Based on that, it is permissible for a woman to dye her hair with whatever kind of dye she wants, unless it is black that is used to cover grey hair. That is not permissible, because the Prophet (blessings and peace of Allah be upon him) enjoined covering grey hair, but he said, “Avoid black.” And this dye should not be something that is unique to disbelieving women, in the sense that if this woman is seen, it might be said that she is a disbelieving woman, because no one uses this kind of dye except disbelieving women. In that case, it is prohibited for the woman to dye her hair with it , because the Prophet (blessings and peace of Allah be upon him) said: “Whoever imitates a people is one of them.”
If the dye is free of these two elements – meaning black that is used to cover grey hair, or a dye that is used only by disbelieving women – then the basic principle is that it is permissible, so the woman may dye her hair however she wishes.” (Fatawa Nur ‘Ala Ad-Darb, 2/22)
Shaykh Ibn ‘Uthaymin (may Allah have mercy on him) was also asked:
Is changing the colour of the hair with the chemical dyes that are available in the market prohibited?
He replied: 
“With regard to changing the colour of the hair from white to black, this is not permissible, because the Prophet (blessings and peace of Allah be upon him) instructed us to avoid that, and a stern warning is narrated from the Messenger (blessings and peace of Allah be upon him) regarding the one who dies his grey hair with black .
As for changing the colour of the hair to any other colour, there is nothing wrong with that, because the basic principle is that things are permissible unless proof is established that they are not allowed. This applies unless this is done in imitation of disbelieving women, because that is not permissible, as imitating the disbelievers is prohibited, because the Prophet (blessings and peace of Allah be upon him) said: “Whoever imitates a people is one of them.”
Moreover, the questioner stated in her question that it is chemical. Based on that, reference must be made to doctors concerning that, to find out whether this dye will cause any harm to the hair and scalp. If that is proven, then it is not permissible to use it.” (Fatawa Nur ‘Ala Ad-Darb, 22/2)
And Allah knows best.