Emailed lush and thought I would post the response:
As you know grey hair is always more resistant to colour and a bit harder to completely cover.
There is a way to get a deep coverage with the Henna. Henna works as a tinted varnish that builds up layers of colour on the hair. To cover grey, you would need to first put a layer of the rouge Henna as a sort of under coat, and then the next day, apply the Brun henna. As your ends are already darker, I would recommend using the rouge just on your roots first. then the brun just on your roots, then brun all over your hair. I know this sounds like a lot of hard work, but it does result in a beautiful colour that protects your hair and gives it shine and body.
We advise customers to wait between 6 to 8 weeks before applying henna to chemically treated, coloured hair and we would also advise performing a strand test first.