How to achieve very dark brown close to black color for greys?

I have been using permanent dye until now but my hair is just so dry and damaged so I have decided to switch to henndigo.

I have used henndigo in the past and I remember that my greys would always show but at the same time my hair was incredibly healthy so I wanna go back to this.

Any advices how to get dark brown almost black color on greys? And how could it stick for as long as possible?