The mix I used was about 50 grams of henna leftover from a previous henna hair dye, which I had frozen. It was originally mixed with a mildly acidic liquid to make a very think paste, left overnight until I saw dye release, and then thinned with more juice. I simply thawed it this time) and applied it to my hair. After wrapping in plastic wrap and then a towel, I left it on for 4.5 hours.

This is a picture of my gray roots after my second, half henna and half indigo. This is just what I wanted, a cool brownish-red!

On the darker parts of my hair I got a dark auburn / dark ash brown depending on the lighting. On the gray roots I have streaks of a light copper/brown color.

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
Amla: https://www.mehandi.com/Ancient-Sunrise-Amla-Powder-p/aml_01.htm
Henna time and temperature: https://www.tapdancinglizard.com/mixing-and-testing-your-henna-mix pp. 10-13
Henna color oxidizing ("rusting"): https://www.tapdancinglizard.com/mixing-and-testing-your-henna-mix/ pp.25-29
Chemical hair dye allergy: https://www.tapdancinglizard.com/henna-for-hair-2/ pp. 18-26

Video How-To
Going red with henna: https://vimeo.com/manage/videos/739464950
How to mix Ancient Sunrise Henna: https://vimeo.com/manage/videos/770522457a>
Henna Root Touch Up: https://vimeo.com/manage/videos/767016457