Get two red objects of similar depth: one a blue-leaning cherry or fire-engine red, one a warm tomato or scarlet. Hold them one at a time under the chin in daylight. Then follow the one instruction that makes this work — do not look at your face. Look only at the flat strip of skin immediately below your lower lash line, between the lashes and the top of the cheek. That is where a temperature error shows itself first, because the skin there is thin, has blood close beneath it, and offers nothing to hide behind. With the wrong red, a cast appears in that strip within a second or two: a lavender or gray-violet on one type, a khaki or yellow-gray on the other. With the right red the strip goes clean and the whole under-eye looks less tired than it did a moment before. The reason for two reds rather than a red and a blue is that red at full strength is a color both of these types can carry, so you have taken clarity out of the comparison and left only side, which is the only thing in dispute.