Dress & Attire

Using dark brown hair dye

Q: A man's wife is seven years younger than him and his beard hair is turning white, whilst her hair is black. Is it permissible for him to use dark brown dye, which makes the hair look black, provided the ingredients are halaal and the dye does not coat the hair and the water reaches the roots? If this is not permissible, what other products and colours can be used?