I have had grey hair (I have dark brown with reddish undertones) since I was in my late teens.
I have a couple of hairdresser friends and have asked their advice about home dyeing and it's worked well since then
He said buy professional products from Sallys, or online (eBay sells it)
Get a mixing bowl, a measuring pot and one of those fanned brushes. Have to
Buy 9 pct peroxide and majirel colour. Get one in a base colour (I believe I use 40) and one in a warmer colour (I think something like 4.8 or 4.6 or 4.4) You need this warmth as the redder dye molecules are the quickest to fade so need putting back in, as it were. Also you need some warmth in hair colour or else it looks flat.
Have on hand a knackered towel and some vaseline and some grips/hair band.
I basically count on using a half tube of the base Majirel (costs about seven quid) and a quarter tube of the wamer shade for a root retouch. Mix with requisite amoutn of bleach (it's something like 3:2, but the leaflet will tell you) and it should be quite stiff and firm (home dyes are way too runny I find). Apply mainly to the bits you can see -- usually crown, sides and temples and then try to work through layers. It can help to have divided hair in 4 sections with sections grips.
ONLY APPLY TO THE ROOTS. LIKE ABOUT A CENTIMETRE. This is proper professional hair dye and you will overdye it by running it through to early. In fact I hardly run it through nowadays. I had too many experiences with blackening hair as I was overlayering the dye and turning my hair into straw.
Put Vaseline around ears and hairline, but don't worry too mcuh, any staining will fade and it's better to catch the hairline greys than miss them if you are bothering to dye,
Be very careful the timing starts from when you apply it (ie, don't dick about for 15 mins applying it and then time from then). You can run the tip of the brush along the grey bits to see if it's taken. You can also lock the dye in better blasting it with a hair dryer.
About 5 mins before the end you can run it through the rest of the hair but be careful not to overprocess.
Rinse and condition, although I use my own stuff as I do the Curly Girl method.
This dye will last until regrowth, but don't be surprised with 30 pct plus grey you have to do it every 3-4 weeks.
More expensive than chemists' jobbies on special offer, but not that much more, and much better value than a hairdresser which can cost £40+ for a single full head colour.