Let’s take a look at the color wheel.

There are three primary colors: yellow, red, and blue.  These are original colors cannot be created by mixing different colors.  Primary colors are mixed together to create secondary colors. 

Color related to hair:
Every level of hair color has a different undertone.  This is the color that is exposed after hair is bleached.  Brown hair tends to have a red/orange undertone while blonde hair has a yellow undertone.

Color Correction:
Opposite colors on the color wheel is used to eliminate unwanted color and tones.  For example, in order to remove yellow brassy tones from blonde hair, purple shampoo or toner is used.  If brassy hair is more on the orange side, products with blue pigments would be used instead.  Red can be used to get rid of green tones in here.  Using opposite colors on the color wheel will result in neutral colors.