All About Hair Color
If you know the color wheel it becomes easy...
It may sound a bit odd but your hair is a mixture of 3 colors: red, yellow, and blue. These are the primary colors. Secondary colors are orange, green and violet. If you look at the "wheel", a color opposite (directly across) will "negate" that color. This means that if your hair is an orange color, green will make it a brown color. If your hair has a yellow tone, violet will cancel it out.
One of the most important elements of hair coloring is determining the hair's "underlying pigment". When you choose a color in a swatch book, your hair may not come out that color because of the underlying pigment in your hair.
Underlying color + Artificial color = Final result