This post is for color correction mostly.

All my life I thought I should use light beige concealer to correct my under eye circles. I used it and there were no effect except yellow color under my eyes and to cover the blue circle I have to put more concealer on it which was very bad.

Later I've found out that I have rosy-pale skin and I need pink or salmon color concealer and that definitely saved my skin from too much product and covered all the blue circles under my eyes.

To conceal all the imperfections you should definitely know its color and which color will cover them even with a small amount of product.
And this is where the simple color wheel takes place.

* Except pale skin and rosy-pale which should be covered with a little different colors.

So when you mix some complimentary colors you can get a neutral gray which you can later conceal with a normal color of your skin or a little more light color of your foundation.