Colouring grey hair

What colour is the hair that isn't grey? I have a lot of grey, but my natural colour is darkish brown, so I still bleach my roots. You could just do a bleach bath, but you might get a not-so-pretty fade if you are going blue. When I had red hair I could get away with a very light bleach, as the red blended the grey and bleached to ginger hairs beautifully. When I went blue I had to bleach my roots fully to get full coverage.
I have yet to find a colour that takes on unbleached grey hair, it is so resistant. A peroxide only soak (AKA pre-softening) does help the dye to penetrate, but I never got fully saturated colour only pastel versions.

Many thanks apart from the grey roots my hair is completely bleached light blond and covered in elec blue with turq bangs. It was bright red before this but l had to do several bleaches to get it blonde. I assume most of my hair is now completely grey coming through. A bit worried about overlapping the bleach onto previously bleached hair as dont want too cause anymore damage to it.

Sorry, I meant your natural colour. If it is mostly white you can just do a quick bleach bath, but obviously a quick bleach bath will turn any dark hairs red or ginger which is not a great base for blue.
Make sure you bleach over coconut oil to minimise damage, and if you do your roots before they get too long the heat from your head will help the to process more quickly, so you can get away with a lower vol of peroxide which is less damaging. Having a friend to help is the best way to avoid overlap.

Thanks for the help xx