Firstly apologies if you think after reading this that we think your stupid but we thought to share a few hair facts with you and colouring to make things a little more simple to understand. There are some colours that will fade, tone fades and it is possible however fading in a solid line is not going to happen its actually called regrowth.
When you first started colouring your hair to cover the odd grey hair, life was fairly simple, you got it done every 6-8 weeks and even then hardly noticed the odd white you had, now a few years later there is more than a few “odd” grey hairs its more like a lot and your dark color that you were 10-20 years ago that you insist on still doing looks black against the white hairs as it grows. The easiest way to explain it is brown will look black when you put it beside white, and a few white will look like a bunch beside brown or dark hair. Your hair is constantly growing, so if your have white hair even colouring over it it is growing from the moment it is covered, the darker the color the more visible it will be as it grows. If you expect not to see any white for weeks then you are delusional, its like a guy saying I shaved a week ago and now I have facial hair, it literally grows 24/7 and you will see small dots of white within days, this is normal and reality.
Easy way to get longer out of it is simple, do not take your color as dark as you are, as you get more grey ( or white hair ) the take the color slightly lighter so there is less variation. Add some highlights to your color to break it up and its not so solid.
Color does not fade in a perfect line, its called regrowth!