Hair creme developer is the chemical added to hair color that helps the color to oxygenate and mix together to create the desired shade. The developer is what helps the color to attach to the hair shaft, lift color for lightening or add color for darker pigmentation of the hair shaft.
Creme developers have identifying numbers from 10 to 40, with the higher numbers performing faster lightening than the lower numbers. The numbers do not refer to the tone of the hair. When lightening hair, it is not advised to use less or more developer than required. This will affect the results and could negatively impact the end results.