Hair colouring advice needed please

[kipperydippery]

I am early 40s. I have long, very thick, curly brown hair. The grey hairs have gone from a few to quite noticeable.

My dream is to find a way of getting the grey hairs to somehow be reddy auburn highlights. Henna shampoos have failed :(

I don't have the spare cash for a hair salon to do it for me every month.

I have some experience in colouring my hair, I spent my late teens & early 20s turning it every colour you could think of, but I'm at a loss as to where to start with this.

All suggestions & advice welcome.

I wonder if choosing a light red shade and applying it all over would work? If its lighter than your natural brown, it won't show on the dark bits but will be noticeable on the grey strands.
I'd go for a semi permanent colour just in case you don't like the effect, but make sure its one that covers or blends in greys.
However if you have a concentration of greys in one place,might not be such ba good idea, as it will go orangey.