According to Salonfree, pick the strength of peroxide that provides the right amount of lift to mix with hair color. The hair color manufacturer provides information on the packaging that explains what level of peroxide is needed and how to use it.
Peroxide, when mixed with hair color, is used to lift the melanin in hair, creating a lighter color. Peroxide is made in cream or clear form. Each form provides different consistencies for the hair color and comes in four strengths: 10 volume, 20 volume, 30 volume and 40 volume.
Level-10 volume is used when the hair color is changing but remains the same level, such as turning light brown hair into light golden-brown hair. Level-20 volume is the standard level of peroxide and is used with permanent hair color. Levels 30- and 40-volume peroxides are used for high-lift hair coloring.
Schwarzkopf Professionals explains that when equal amounts of peroxide and hair color are mixed together, the strength of the peroxide is halved. Most hair colors require at least 1.5 percent peroxide to develop pigments in the hair color, and any remaining value in the peroxide creates the lift. To determine the amount of lift, multiply the percentage of the peroxide with the amount being used. Divide that number by the total volume of hair color and peroxide used together. The resulting percent is the strength of the peroxide. Any amount over 1.5 percent is the amount of lift.