Color Reverse gently reverses the last oxidative color process (color mixed with developer). When color is mixed with peroxide, the color oxidizes and swells in the hair shaft to create new color. Color Reverse shrinks those color molecules so they wash out of the hair. Color Reverse has no ammonia or bleach so you can re-color the same day. Because it has no bleach or ammonia, it is odorless. As with any color removal system, black will always be the most difficult color to remove.