Often times skin tone and undertone are used interchangeably. When using skin tone it typically refers to whether someone is light, fair, medium, or dark. Undertone on the other hand, usually mean being cool or warm.
Cool undertones are usually revealed in fair skinned people, those who appear more pink than peachy, dark brown skins with red tones, skin that looks bronze or brown when tanned. While warm undertones are reflected in people who can easily Tan themselves, who appears more peachy and with visible yellow tones.