Hair developer is mixed with color in order to activate the color and help it penetrate the hair shaft. Without developer, color would have no effect whatsoever on hair. It would lay on the outside of the hair shaft only to be rinsed away with the next shampoo. There are two main types of developer: creme and liquid.
Creme developer has a thicker consistency and is often used for tricky applications where the color must stay in place. Color that has been mixed with creme developer can be combed into place with no fear of runs or drips.
Color is activated when developer is added. Both color and developer can be stored separately for up to five years since they do not react until mixed together. Once mixed, their effectiveness expires after one hour.
Developer is the chemical that is responsible for lifting the cuticle layer of the hair, allowing color pigment to penetrate the hair shaft. The cuticle layer is made up of tightly closed, layered cells.
Creme developer is available in many strengths, called volumes. Low-strength developer deposits color by lifting the cuticle layers just enough for color pigment to slip inside.
High-strength creme developer opens the shingles of the cuticle layer wide enough for natural color pigment to be removed from the hair.