Your undertone is the color that shows through from under the surface of the skin and does not change with seasonal sun exposure. This does not include any redness you may see after a workout or a hot shower. Look at your overall tone including your body.
Cool undertones have shades of pink, red, or blue. Typically, those that have a cool undertone will have blue/purple veins and prefer wearing silver jewelry.
Neutral is neither cool nor warm. If your veins appear both blue and green you may be neutral and can usually pull off silver or gold jewelry.
Warm undertones have yellow, golden, or peach undertones. Those with warm undertones have green veins and gold jewelry complements you the best.