Vein test
Looking at wrist-vein color, green reads warm, blue reads cool, a popular but scientifically weak undertone test.
The vein test is a widely circulated home method for guessing undertone: look at the veins on the inside of your wrist in daylight, and the folk rule says greenish veins mean a warm undertone while bluish or purple veins mean a cool undertone. It is popular because it is fast and needs no equipment, but color-analysis research rates it as low reliability, because vein color actually depends on how the skin and the layer of fat beneath it scatter light, not solely on undertone; veins set deeper under the skin read bluer simply because more tissue sits between them and the eye, regardless of the person's real undertone. The test performs especially poorly on medium and deep skin tones, where veins commonly read green no matter the underlying undertone. Color analysts generally treat the vein test as, at best, one weak data point to be checked against stronger indicators like the white-paper test or a side-by-side comparison of warm and cool drapes, never as a standalone answer.
Why it matters
The vein test is often the very first thing people try, and its documented unreliability, especially on medium and deep skin tones, is exactly why relying on it alone leads so many people to the wrong undertone conclusion in the end.