Removing Redken color brews

[Cissorhand]

I have a client that has been growing her level 4/5 Hair Color out. We have been using Redken color brews to camouflage the gray for the last year and a half. I have left it on longer than 5-10 minutes during the application to get it to last longer. I'm wondering what would be the best way to remove the color brews using Malibu C products hopefully if they would do the trick. Or if there is a better way I would be so curious to know! I appreciate your advice!! Thanks so much!!