My natural hair color is dark brown. Although I have used chemical dyes for many years (which I stopped a year ago), no strange color transformations occurred after henna use.

I used 300 grams of henna powder and mixed it with a mildly acidic liquid. After several hours, Teemu applied it to my hair using silicon gloves. We used a little of henna that remained from Teemu's hair to do some reddish strands.We were a bit afraid to see the result but it came out really cool. Both of us kept it on the hair for about 4hours before rinsing the paste off.

I used 200 grams of indigo powder mixed with water and applied the paste to my hennaed hair.

My hair is softer and so shiny!

Even after two months Teemu's color is still very strong and bright!

Notes and References
Henna: https://www.mehandi.com/Mehandi-com-Ancient-Sunrise-Henna-for-Hair-s/121.htm
Don’t have fruit juice on hand? Keep some fruit acid powders in your cupboard.
Mildly acidic liquid: https://www.mehandi.com/Fruit-Acid-for-henna-dye-release-s/299.htm
More info on mildly acidic mixes: https://www.tapdancinglizard.com/henna-and-acidic-mixes/
Ancient Sunrise henna for hair kits with everything included: https://www.mehandi.com/Mehandi-com-Ancient-Sunrise-Henna-for-Hair-Kits-s/124.htm

Video How-To
How to mix Ancient Sunrise Henna: https://vimeo.com/manage/videos/770522457
Indigo Touchup: https://vimeo.com/manage/videos/358899510