Color & Developer Ratio

A: When mixing color formula with a developer (whether a cream developer or liquid) the ratio of 1:1 means that you combine equal parts.
The bottom line is you just want to use equal measure of hair color to developer to ensure best results.
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See also:
How to dye your hair
What does the hair color developer do?
Can most developers be used with other products?
Is there a difference between hair developer and hair activator?