ditto what mrsmoscow says, except i got significant greys at 19.
i used to pay hairdressers $$$$ for one colour hair dye that with a bit of care I could do myself. It's not highlights or anything tricky.
Also, although I use chemist boxes (such as Nice & Easy), I don;t find they last the full 4 weeks. I am grey at the temples so regrowth really notices.
a couple of tips, go lighter rather than darker. Go 5.3 rather than the 4s. This is because being an amateur you are likely to overlay the colour and over time you will get dark, blackish buildup.
I buy my majirel from ebay - it's usually about 6-8 pounds a tube. Any old 20 percent peroxide will do.
I tend to use a third of a tube with a bit of the warmer shade mixed in each time. I measure the 50:75 ratio of dye to developer with kitchen tablespoon measures in a hairdye bowl with a tint brush (pennys at a Sallys)
Apply from visible greys onwards. This stuff stains so protect floors and skin with vaseline (a certain amount of scalp dying is inevitable and will wash in two washes) Time it and don't go over the time. It will overprocess.
I also find you can use an old bottle saved from a chemist brush but it can sometimes be too thick. I personally don't bother and just use the tinting brush
I can't easily get to Sallys (two children under three) but the big ones have helpful staff (Shaftesbury Ave I recall was good) who are in the business and who can advise.