I use henna, specifically this two step one
here 🔗 [Link: evitamins.com/hennagray-red-light-mountain-28870/].
The advantage of henna is that it binds to and coats the hair, makes it stronger and improves its condition, plus its effect builds with each new application. I have very fine light brown hair and the pure red makes my hair go a sort of coppery colour (at least after it's oxidised over a few days). This two step henna process definitely covers grey and doesn't wash out, though obviously roots tend to be grey as hair grows. Other shades of darker/less red henna tend to have indigo mixed in with them. Indigo is another amazing natural colourant, but my hair just sucks it up and turns very dark brown/black, regardless of the shade of the henna mix, though many have great success with it.
btw, a lot of the girls on the
longhaircommunity.com forum, and who have amazingly long and beautiful hair - swear by the henna product called: Rainbow Research Henna. I've never used it 'cos am happy with the two step brand I use, but if you're going to try henna, could be worth bearing in mind.