I am (and have been sadly) going grey since my early 20s. I'd say I was 40-50 percent grey now (39).
I have over the years tried many dyes and regularly had salon dyes but now I have a toddler and another baby about to come I do not have the time to go and have my roots done every 4 weeks.
I used to do this and recently asked my hairdresser for a refresher, as I'd lived abroad and had a hairdresser do this for me.
Basically home kits won't be sufficiently strong to last more than 3 weeks. With proper salon dyes you will be able to go even with dark hair to 5, maybe 6 weeks.
I am dark chocolate brown so my colours refer to this and my percentage of grey. You can buy the products on the internet, or visit a Sallys (the staff will help). I buy my dye and developer on eBay as I find it hard getting to Sallys.
But I recommend you either consult a hairdresser -- go in for a dye job but say you want to recreate it for roots at home as you can't make a hairdressers every 6 weeks or so, they'll tell you what to use.
Majirel -- the best permament to colour grey (in my hairdresser's opinion). I use 50 percent 4 (base brown) and 50 percent 4.35 (that's a warm golden brown). I then use 6 percent developer. I measure it out with a tablespoon measure (one each of dye, then I think 1-1/2 of developer, although the leaflet will say the ratio). Mix in hair dye bowl with tinting brush (available for pennies from a Sallys)
Use proper latex gloves.
Apply to roots only (if roots is what you are doing) and no further up the hair shaft. Use the thin end of the brush to separate and apply, paying attention mainly to the areas you see and temples etc. If not that dextrous you could put the mixture in an old hair dye home kit bottle which has a nozzle.
Try not to overlay the colour on old colour as you will just build up and build up and get either too dark or too ashy/one toney.
Once done, take this as your timing start. Again, the leaflet will give the time - I think it's 35 mins. Then add a little water and take the colour through the rest of the hair to even the tone.
This is roots, virgin whole head you just apply all over. If ends have lost colour take the rest through sooner.
Basically a home dye doesn't penetrate as far (they need to be weaker otherwise unhappy punters would sue them for mistakes!) into the hair shaft as the developer is usually weak. A professional hair dye/developer gets the molecules right in there.
Go once and get it done and watch how they do it, and ask them what they used is the best way.
Where I am a root dye costs 40-60 pounds and grews out in 5 weeks (dark hair, high grey percentage) whereas this costs me 4 quid a go.