It is a Sunnah to Dye you Grey Hair.
Dyeing grey hairs is Sunnah and is part of the teachings of Islam. It means dyeing grey hairs on the head and in the beard, for men. For women, it refers to the hair of the head.
Islam Q & A has a detailed Fatwa about how and why a Muslim should change the color of their grey hair. Fatwa Number: 7227 can be read in full detail here: http://islamqa.info/en/722
Mentioned in the four books of Sunan (Abu Dawood, Al-Tirmizi, An-Nas’ay and Ibn Majah), it is narrated that Prophet Muhammad (peace be upon him) said:
"Henna and Katam are the best of what you use to change (the color of) white hair".
In Sahih Muslim, a Hadith is narrated that disallows dying the hair black, when Abu Bakr's father was brought to the Prophet with all his hair being white. Prophet Muhammad (peace be upon him) said at that time: “Change this white hair but avoid the black color"
Prophet Muhammad (peace be upon him) disallowed dying the hair black. But if Katam is mixed with Henna, there is no harm in it. Katam and Henna dye the hair between red and black colors, unlike Wasmah (woad), which turns the hair black.
Imam Ibn Qayyim Al-Jawziyah said in his book ‘Healing with the Medicine of the Prophet’:
“What is prohibited is making the hair pure black, but if something else is added to henna, such as katam etc., there is nothing wrong with that. Katam and henna make the hair a colour between red and black, unlike wasmah (woad leaves) which make the hair black as coal. This is saheeh. (Zaad al-Ma’aad, 4/336).
(Wasmah or woad/indigo is a plant that is used for dyeing). Hence we know that katam/buxus and wasmah/indigo two dfferent plants, are not to be used on their own, because it gives a pure, coal-black colour, but it may be used with henna to give a black colour with reddish highlights.”
Pure, Organic Katam also known as Buxus is extremely hard to come by nowadays. Here at oh so halal, the closest to Katam that we can get our hands on is Wasmah, also known as Indigo. Our indigo is 100% Pure, Natural, and Chemical Free. Indigo behaves like katam so that it will turn henna brown and almost black depending how long you leave it in for. Remember to avoid ending up with a black shade!
According to the above ahadith, my understanding of dying the hair with Wasmah/Indigo alone is prohibited in Islam as it will dye the hair black. Mixing Henna with Indigo however, is permissible as it will give you very dark hair with a reddish tinge to it so that it doesn’t color the hair pure blIndigo is a plant that has a blue dye in it. You probably have indigo dyed cloth in your closet, in a pair of blue jeans. Jeans are dyed with indigo. People have used indigo alone or in combination with henna to dye their hair black for at least 4000 years. During the 19th and 20th century indigo was such a common hair dyin Europe and the US, that it was marketed as "black henna". Henna is never black, henna never dyes hair black, but powdered indigo resembles powdered henna and it DOES dye hair black. So, when indigo was sold as hair dye, it was called "black henna". When it was sold to dye cloth, it was called indigo