I had been considering buying Colour Wow root camouflage powder for a while after seeing you tube demonstrations. The stuff looked great. My hair is coloured a very dark black/brown and my greys come through within 12 - 14 days. My grey is dominant and outnumbers any naturally dark hairs strongly.
I spotted on Amazon what looked like a cheap version. It's called W7 Cover Up Root Camouflage in dark brown for £4.97. It came within a few days and it's AMAZING!. It only takes me 2 minutes to to dab some along the most visibly grey rooty bits such as temples and parting. It holds all day until I wash it out and I don't believe it looks like anything other than dark hair. I managed to extend my colouring cycle by a week by brushing this stuff on each morning.
It comes in other colours from blonde to black but I've no idea if they are as good. Just thought I'd post as I'm really pleased with it, especially as my hair is pretty thin at my temples so when the grey comes in it looks much worse as my scalp is white and the white roots make this area look more sparse. The powder clings to the hair and the skin so it really does camouflage it for me. This helps if I tie my hair back exposing my temples which I was avoiding before.
Sorry, that was much longer than I meant it to be, just thought this might help someone!