I covered my gray with Natural Instincts Costal Dune color. It won't wash out and now I have gray roots. I'd like to go back to my gray color. Am I correct that I can use the Color Zap and not have to apply new color? Somebody told me that color removers would just turn my hair orange and I'd have to apply new color.

Mellba, a color remover is used when artificial color needs to be removed from the hair. Color Zap can be used to remove permanent color and prepare for immediate re-coloring. Color removal products do not discriminate between artificial and natural hair color. You can end up with orange or yellow hair if you don't follow the instructions precisely. Hair color correction can be very tricky and it's always best performed by a professional.