I have dark brown hair and would like to know about the formula needed to retouch my roots and cover grey at home as I hear box dye aren't recommended or good for the hair and don't always have time to go to the salon to retouch.

I get a good amount of compliments on my haircolor, and it is amazingly inexpensive. I use organic mountain henna, which is under $10 a box, and allows me about 4-5 root touchups (I do that about every two and half weeks) I'm 46, so likely do have a bit of gray, but it seems to cover it all well. I've read for very white/gray hair they sell a pre-henna treatment (also henna I think but used as a neutral hue base coat before you choose your preferred color).

The color I use is bright red (which looks natural, not a punky red), but there are tones that are simply different shades of brown up to black. I like it because there's no toxic chemicals, and it doesn't strip the hair follicle. I think it's marketed as semi permanent, but I find it pretty much permanent. When I do the roots, the wash out seems to refresh the rest of the color nicely. I also use it on my eyebrows.

I mix it with warm water and coconut oil until it has a muddy consistency, put gloves on and just massage it into my roots and hairline, wait about a half hour (or more if I have time) rinse, wash out and condition. For brighter enhancement I sometimes use Aveda Madder Root color conditioner (they have a variety color conditioners for about 20 bucks that help keep the color strong, but it's not necessary--the henna does it's job nicely).

Just DON'T use any metal in the mixing of the product-it can affect the color and possibly turn it green. Or so I read. I just use a plastic spoon and cup.

Extra note: I just reread your post and you wanted something between appointments. Henna in that case isn't the best as I don't think it is good to mix with other types of chemically dyed hair. That said I did transition out of chemically dyed hair with henna and didn't seem to notice much problems. Probably because I let a good amount of inches grow out before I did so.

Henna is great as 100% color maintenance though. At least that's been my experience.

I do see at salons sprays and powder compact boxes that you apply root touch up powder as a cosmetic coverup. My sister has that for her salon platinum colored hair, and she likes it.

I will try it out, appreciate your help!

What color do you recommend for me?

Brown or Red

I would highly recommend that you see a professional.

I would highly recommend NOT using henna dye on your hair. If you use any other dye like at the salon after your hair will completely melt off from the line of demarcation.