Human eyes evolved to see in color largely for the purpose of detecting skin color changes such as when other people blush, Changizi said. These emotive skin color changes are extremely apparent because humans are hard-wired to notice them, and because the background skin color remains unchanged. The contrast against the nearby "baseline" skin color is what makes blushes so noticeable, he said.
Human skin also changes color as a result of hundreds of different medical conditions. Pale skin, yellow skin, and cyanosis – a potentially serious condition of bluish discoloration of the skin, lips, nails, and mucous membranes due to lack of oxygen in the blood – are common symptoms. These color changes often go unnoticed, however, because they often involve a fairly universal shift in skin color, Changizi said. The observer in most instances will just assume the patient's current skin color is the baseline color. The challenge is that there is no color contrast against the baseline for the observer to pick up on, as the baseline skin color has changed altogether.
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