I am going to try to make this short, but get my whole story out so I can hopefully get the best answer. Most of my life, my natural hair color was dark drown and I always highlighted it. 4 years ago, I was tired of messing with highlights, so I had my stylist (of 21 years) color it back to as close to my natural color as possible. (using a 3) Well, he passed away & after multiple failed attempts of trying new salons... I just decided to save the money and do it myself. It worked for a long time. Before this last time, I waited a lot longer to cover my roots and when I did, I noticed my natural hair color has started coming in a lot lighter... dark blonde, almost matching a #6 color swatch. I went ahead with my normal process, permanent 3N and it’s been about 4 weeks. Roots are already showing again and I want to start using a 4 or 5. My question is, what is the best way to get the rest of #3 out so that all of my hair will be the same color when I use new color? I've read the stories about prell, dandruff shampoo & dawn... but most of those seem too geared towards semi-permanent. Any advice is greatly appreciated.

*Forgot to add that I don't want to bleach or strip my hair. And the color I have been using is from Sally's Beauty Supply with a 20 developer.

I've tried Prell and Head&Shoulders, neither has worked, any other suggestions are greatly appreciated.