TROY The eyes may be the window to the heart, but the skin is the doorway to your state of mind. The skin turns red with embarrassment, pale with fear, blue with the cold. A change in skin tone can also indicate a health problem, and a professor at Rensselaer Polytechnic Institute here has devised a way to help doctors and nurses better judge changes in skin color. Mark Changizi, an evolutionary neurobiologist, suggests putting patients in gowns or using stickers that match a patient's healthy skin tone. That way, a nurse or doctor has the original color to compare the skin changes. 'You have to have that baseline nearby to see these color signals on the face,' he said. Changizi published an article about his proposal in January in the journal Medical Hypotheses. The idea is an offshoot of his main research on why some animals evolved to see color. Few animals have color vision, with the exception of some primates -- like humans. The RPI researcher has challenged long-held beliefs that color vision evolved to help primates find fruit in trees and distinguish edible vegetation from harmful plants. In his book 'The Vision Revolution,' Changizi theorizes that color vision evolved to help primates as a social tool. 'It's about seeing the moods, the states, the emotional expressions in others,' he said. Indeed, his research found that primates that have bare skin -- faces with no hair, bare rumps -- were also the ones with color vision. Among other things, recognizing changes in skin tone helps primates discern whether their partner is in the mood for sex. Many of these changes to skin color are driven by blood flow and the oxygen level of the blood, so Changizi started thinking about ways to use this information in the medical field. 'The eyes are still a very important part of medicine. To this day, (skin pallor) is still mentioned in 10 percent of diseases,' he said. For example, lack of oxygen in the skin -- called cyanosis -- is an indicator of lung or heart problems. One of the main symptoms is a bluish discoloration of the skin, lips and nails. While humans are good at detecting color changes that happen before their eyes, they are not good at judging changes that they did not watch, or that occurred slowly or uniformly across the skin. Using patient gowns or stickers as a baseline overcomes this problem, Changizi said. Changizi illustrated this with five squares of color placed on a bright blue background. Against the blue, all the squares appeared peach. But when the blue was replaced with one of the peach squares -- the baseline -- the remaining squares looked completely different. Some appeared bluish while others looked yellow -- even though they were the same color as the first illustration. His system would also help clinicians see skin tone changes in black, Hispanic and patients of all colors. Changizi's proposal could help supplement pulse oximeters, which are small devices placed on a finger to measure oxygen level. Changizi's system could be used in areas of the hospital where patients have not been hooked up to a oximeter or in third-world hospitals that can't afford oximeters. 'The trick is,' he said, 'if you want to harness the oximeters in our eyes for clinical purposes, you have to put a baseline there.' 