Yes you can. Will cover grey hair an orange red almost copper brown colour. To get various shades of red use henna only but to get various shades of brown (light, medium, dark) and black use with indigo powder.
As long as time permits but at least 2 hours. The longer the more beneficial to hair and colour.
This is pure natural henna powder which turns gray hair orange red or existing brown hair into auburn. It is not chemical herbal henna which contains ppd to give instant colour.
If hair is:
Blonde – vivid or bright red
Black – black with red glow (over time)
Gray – copper red
Medium Brown – auburn
Dark Brown – dark auburn
Red – vibrant red
Any acidic liquid for longer lasting colour such as lemon, apple or orange juice (all can be diluted with water), tea (preferably chamomile) or coffee. Some people use apple cider vinegar.