An undertone falls into three categories; cool, warm and neutral. The best way to determine your undertone is to examine the color of your veins on the underside of your arm. If your veins are blue, your undertone is cool. If your veins are green, your undertone is warm. If you are veins are a combination of both colors, which may or may not include yellow, your undertone is neutral. Once you discover your undertone, you will be able to choose an appropriate foundation shade in that particular family.