Lilium Cream Developer is a product that's mixed with color to activate it and enable it to penetrate the hair cuticle. It contains hydrogen peroxide, and it either lifts or deposits the color.
You need to mix this developer with a basic ingredient such as a hair color or bleach to activate it.
Lilium Cream Developer allows the color to produce your expected outcome by opening the cuticle for the color to infiltrate. It also determines the resulting color; hence, you should choose the right developer.