My hair color I use black cherry with 20 vol. Leave it on for 45 min. but it never stays dark. Always back to same. Is there some filler I can use to keep the darkness?

How long after you color does it revert to the natural color?

A week at most.Thinking of trying 10 vol

Is this a box color bought at a big box store?

If not, are you using the right developer? Is it semi-permanent, Demi, or permanent?

Are you doing this on wet or dry hair? Is your natural hair grey?

It's wella from Sally with 20 vol developer that goes with it.I did hair a few years back but I know things have changed.