Color remover question

Okay, I posted on here a couple days ago about getting rid of my currently fading hair. After the advice that was given I decided to go buy some color remover (one n only color fix). I wont be applying for a week or so, want to do some coconut oil treatments first but my questions are...
    Since my roots have grown out significantly should I be applying this to my entire hair regardless of the regrowth?  Or just apply a couple inches below my scalp and only apply where the faded color is? Lastly if I did (while applying) accidentally get the color remover on the regrowth would it effect that virgin hair color at all? Because the last thing I want is for my hair to be a billion different shades after I do this. I'm anxious to get started and try it out but nervous! 😄