Lab Garnier is fine but darker "Nice 'n Easy" stains skin. If you're going to do head as well, start with beard as it requires a little longer. 25 mins is usually enough. Use a proper brush to apply solution and work it into all the roots. Nasty amonia smell gets up your nose and makes your eyes water so do it in large airy room. If you wash and scrub before you apply the dye your skin will not stain, as it seems only to attach to keratin (fingernails and hair) and greasy spots.
The chemical process is fascinating if you like that sort of thing. Try googling or looking on the dye manufacturers' sites.
Greasy spots? I meant areas of greasy skin, not surface eruptions acne-wise. Sorry, no offence meant. Oh, by the way, I know it should be "ammonia" (still waitng for that edit function - AB Ed) and here is a link for how it works.