I read this last week feeling that it was about time I did mine, I have greys sprinkled through my naturally mid to dark brown hair, which I colour light brown.,
Then, on Friday night, shortly before going out, I saw in a very well lit mirror just how much grey was really in my fringe. 
Not a good look, honest.
I had no time for a full head dye so used half a tube of brown eyebrow dye that I'd been sent to review last year. Ten minutes on, rather than thirty and crossed fingers, before washing out.
That worked quite well, being essentially the same chemical formulation as hair dye, at goodness knows what times the price.
I don't recommend eyebrow dye as an everyday choice of hair dye of course, but for those of us whose genes give us Mallen streaks/the piebald look/pepper and salt and heavy on the salt, dyeing with a decent hair dye really isn't such a bad option.
My hair is quite short, so I decant the unmixed hair dye and developer into a plastic bowl and use just half a pack at a time, when I dye my hair conventionally. Carefully resealed, the separate tube of colour and bottle of developer lasts until next time. (Usually 3 months for me, but maybe 6 weeks from now on.). I use Aldi colours, as they total 100 ml of product, and give excellent results.