I'm almost 40, female, and have been going grey since I was 25. I'm still not ready to embrace it, so I've been dying my hair for fifteen years or so. I keep it to my original natural color and my go-to box dye is Revlon ColorSilk in Light Ash Brown, which goes on sale periodically at Target for $3-$4.
I do this about once every three months or so, and I've always liked the results. Here's my process:
1. I like working on 3rd day hair--somebody told me dirty hair takes color better.
2. I brush my hair all out--it's naturally curly so this is something I almost never do.
3. I part it down the middle, throw down a big line of the coloring potion, and then use a medium sized makeup brush to paint it all in to the left and right of the part.
4. Then I take a wide toothed comb and brush the potion down both sides of the part so that it kinda goes into all the hair around it naturally.
5. I work through my entire crown, parting, dying, painting, combing, parting, dying, painting, combing--the grey is pretty intense at the front of my hair and nonexistent at the back, which I totally ignore during this process because the box dye color is a great match to the natural back of my hair.
6. Swab the hairline with a makeup removing cloth to nab any stray potion that got on my skin, go and sit for 45 minutes, and then rinse out and deep condition.
Always looks a little weird (kinda too dark, but nothing embarrassing) for the first three days and then calms down after a wash or two and looks great for six weeks until the grey starts coming back and I have to do it over again.
So I'm posting this partly as a guide for others (I asked my hairstylist how to DIY color my hair five years ago, and she's the one who gave me this routine) and also in case any more advanced hair-care folks want to weigh in.
AND I'm also posting this because as much as I'm happy with the whole process (especially the bargain price compared to a salon!), the one thing I HATE is that for two days after I do this, my scalp is so, so, so itchy and I don't know if it's the cheap-ass dye I use (which in every other way is totally satisfactory) or if I just have a sensitive scalp, but man I wish I knew a hack to make it stop. I tried Sweet N Low and it didn't help at all.