Skin tone is the color of your skin and it depends on the amount of melanin present in you skin. It can change for a variety of reasons (e.g. seasons, lights).
Skin undertone is the color from underneath the surface of your skin that affects your overhall hue. This color remains constant. Tip: check the inside of your wrist. Purple or blue veins often mean a cooler undertone. If your veins look more blue and green you should have a neutral undertone. People with more greenish-looking veins usually have a warmer undertone.