Hair Color Developer
A: The developer in a mixture of hair color does exactly what the name implies: it provides the chemical reaction that allows the hair color molecules to penetrate and be deposited into the shaft of the hair by causing the chemical color to "develop".
The formulation and balance between hair color and developer is designed to use equal parts of each separate mixture of components. Therefore two ounces of hair color which contains ammonia and oxidative tints (usually aniline derivatives) is meant to be combined with two ounces of developer (hydrogen peroxide). This results in the oxidation of the aniline derivatives which creates the color to be deposited into the hair. The aniline derivatives combine with peroxide to form larger color molecules.
When you use less developer than is called for, you will not get as much reaction from the chemicals combined, which can result in duller color results and less penetration of the color. Using more, however, can dilute the color and result in paler results that intended. The use of hair colors, however, becomes subject to an instinct as you work with them.
As you work on a particular client and learn how her hair responds to color, you can find yourself adjusting the color mix a small amount depending on the needs of the situation, but overall you'll rarely vary from the 1:1 color to developer ratio.
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See also:
How to color hair
What is the ratio of color to cream developer?
Is there a difference between hair developer and hair activator?
Is it ok to use a different brand of developer than the brand of the actual hair color?