Understanding Neutralizing makeup colors:
Neutralizing colors are colors that appear opposite on the color wheel. For example- blue neutralizes orange, green neutralized red and so on. So in order to cover red blemishes and blotchy skin, you must neutralize the color first with a green makeup concealer. Then, cover with foundation makeup. By just adding a typical colored makeup concealer, you won't conceal the problem because the red or blue tones will shine through. So you always need to neutralize the flaw first, then cover with your shade of foundation.
Corrective Makeup Color Concealers:
Makeup Color concealers are made typically in 3 color choices- green, yellow, and lavender (mauve). You need to understand which color concealer will neutralize the flaw. Look at the flaw you want to cover. What color is it? Is it acne (red) or under-eye circles (blue)? By identifying this first, you are now ready to conceal those nasty little imperfections!
Quoted from http://styles101.homestead.com/corective.html