I am in the position of going grey, but not simultaneously or drastically. I don't have 'roots', instead I have random strands of grey hair throughout my brown hair - which means root retouchers have never been of any use to me, but I've still been left wondering how best to cover greys without dying my whole head.
Schwarzkopf Hair Mascara has proven to be the answer to my problem. It comes in a plain black tube with pink and white accents. (My only gripe is that I could mistake it for an eye mascara, so I keep it with my hair products so I don't accidentally reach for it while doing makeup.)
The wand is plastic and it makes it really easy to apply the product to those random stray grey hairs. I found the colour (medium brown) to be perfect and it covers the grey completely and doesn't transfer, yet washes out easily.
This product is ideal for seeing myself through the awkward 'grey, but not enough to do anything drastic' stage. Can't recommend it enough.