In most hair color applications, colorists mix an equal amount of developer and color together. This formula is often referred to in parts or as a ratio. For example, the ratio 1 to 1 means to mix one part color with one part developer.
When colorists want to lighten hair by several shades, a service called high-lift color, they often add more developer to ensure maximum lift. A common formula used consists of one part color to two parts developer. Stylists rarely go over the 1 to 2 formula because adding too much developer changes the consistency of the hair color and doesn't offer any real advantages.