Sorry I didn't get back earlier!
My hair is dark brown and it's taken a lot of trial and error to avoid my hair going either black or brassy. Last year I cut off my long hair as it had gone so dark, and I I've managed to keep the new growth a nice medium brown, but you really have to be careful...
Firstly always go one shade lighter than you want to be, if you use dark brown you'll end up with black, but medium brown will give you a better result.
If you like auburn tints in your have a much easier time, but I don't so it can be hard work keeping them out! A lot of people here use Boots own brand medium brown, but I find it too orangy. The one that works best for me is Clairol nice & easy medium brown 118. If you look on the side of the box, there's a graph with cool shades at one end, warm shades at the other, the closer you go to the warm end the more orange in the dye. 118 is the 'coolest' shade they do, and definately the best I've found.
I have tried the 2 new dyes that take 10 minutes Perfect 10 and Excell 10, and while they left my hair really shiney, they did not cover the grays, so I had to redo it all again.
The actual process is easy, and not that messy, so the first time you do it, just follow the instructions. My roots show through after about 3 weeks, and this is when you must be careful. I find the root touch up kits don't cover my greys, so I just use my usual dye (costs the same) to touch up my roots.
For a good result, use a brush.
Be very careful and take your time, only do the roots, not further down the length of your hair. Even though the instructions say to do the roots for 30 mins, then dye the rest for the last 10 mins, I DON"T do that. That's what led to a build up of colour on my long hair.
If you don't mind the slight auburn tinge, ignore this bit! My secret weapon is Aveda Black Malva Conditioner. I mix it 50/50 with the conditioner in the box. I get a purple goo which I put all over my hair and leave for 20-30 mins. If I have time I condition it like this every, or at least every second, time I wash it. This is what makes the difference betwen hair that looks like I dyed it myself, and hair that looks like a spent a fortune on in a salon
I'm probably making this sound way more complicated than it really is, but I am VERY, VERY fussy
If a jobs worth doing, it's worth doing well...