Is it permissible for the woman who is observing ‘iddah to dye her grey hairs?
Question

Is it permissible for the woman who is observing ‘iddah to dye her grey hairs?
Praise be to Allah.It is not permissible for the woman who is observing ‘iddah following the death of her husband to dye her grey hairs, because that comes under the heading of adornment, which is disallowed for her for the duration of the mourning period.
This is what it says in the Sunnah, and the scholars are unanimously agreed on that.
Abu Dawud (3204) and an-Nasa’i (3535) narrated from Umm Salamah, the wife of the Prophet (blessings and peace of Allah be upon him), from the Prophet (blessings and peace of Allah be upon him), that he said: “The woman whose husband has recently died should not wear clothes dyed with safflower or red clay (mishq), or wear jewellery, or dye her hair with henna, or apply kohl to her eyes.” Al-Albani classed it as authentic in Irwa’ al-Ghalil, 2129.
See: ‘Awn al-Ma`bud bi Sharh Abi Dawud.
Ibn al-Mundhir said in al-Awsat (9/571):
One of the things that all of the scholars from whom we acquired knowledge disapproved of [for a recently-widowed woman who is still in mourning] is dyeing their hair.
Those from whom we narrated the view that it is not allowed for the woman in mourning to dye her hair include: Ibn ‘Umar, Umm Salamah, Sa`id ibn al-Musayyib and `Urwah ibn az-Zubayr. We do not know of any difference of opinion among other scholars regarding that. Dyeing the hair comes under the heading of adornment, which is prohibited for the woman in mourning according to our view. And Allah knows best. (End quote)
It says in Sharh Mukhtasar Khalil by al-Kharashi al-Maliki (4/148), in the context of discussing rulings pertaining to the woman in mourning, that she should not dye her hair. (End quote)
Shaykh Ibn Baz (may Allah have mercy on him) said:
As for the things that the woman in mourning must avoid, they are as follows:

She should not apply perfume to her body or clothes.
She should not wear beautiful clothes; rather she should wear clothes that are black, green or blue, and similar colours that are not beautiful.
She should not wear jewellery of gold, silver or other precious materials, such as diamonds, pearls and the like.
She should not dye her hair.
She should avoid using kohl.

That is because the Prophet (blessings and peace of Allah be upon him) forbade the woman in mourning to use all of these things. (End quote from Majmu` Fatawa Ibn Baz, 22/193)
And Allah knows best.