How does root touch up work for hair and anything special for men?

What I am specifically wondering is if the bottle can be reused or do you use it once like hair dye and have to throw it out?

It isn’t for me, it is for a man. I have never used root touch up products. His sideburns are starting to have a lot of grey, and he has about ten stray grey hairs on the top of his head. His hair is very dark brown/black, so to him the grey is very noticeable.

I would never tell him to dye his entire head, because for now his hair is still primarily his natural color, but I have no idea the best product for just dying a few stray hairs.