This pic dates back to 2007 - I'm the one in the middle (sammiched between my hubby and my mom at a 70's party) and that's the hair color I've worn for years. Lately I've been trying to transition to a more brow tone using indigo and henna but I have yet to achieve anything other than my usual red. This time I've increased the ratio of indigo henna to red henna in my mixture.
I buy my henna from a local herbal shop. One of my favorite hennas for brighter reds is called Jamila Henna, but it is somewhat expensive and since I'm aiming for a browner tone, it's too red for my needs right now. There are many different kinds of henna but I strongly encourage you to do your research before you buy as some products advertised as henna actually contain PPD, a toxic substance with many icky side effects. I also recommend not using henna "mixes" from commercial sources, even ones meant for hair. Pure body-art quality henna is the best. Anything else may contain additives that will interact with color-treated hair or may cause allergic reactions.
Here I've mixed about 1/2 cup of red henna with 1/2 cup of indigo henna. This is a HUGE amount, but I have butt-length hair. If your hair is shorter, less than half of that will be sufficient, especially if you are just touching up roots. Henna is an excellent conditioner, so whether or not my ends need the color, I usually apply my henna mixture root-to-ends.
"Black" Henna (Indigo henna, not to be confused with commercial henna containing PPD)
I added the herbs for additional color as well as aroma. You might avoid using anything "chunky", like the cardamom, in your mixture as it can be more difficult to wash out and is hard on drains.
I leave this mixture in a warm place overnight covered in plastic wrap (do not use metal foil, utensils or bowls). I'll apply it with gloved hands because it can stain hands and nails, and then leave it on for a minimum of four hours. Then I'll rinse without using shampoo or conditioner. "After" photos to come tomorrow!