How much cooler are you wearing a white shirt on a hot day rather than a grey shirt?
You could conduct an experiment and find out. It would depend on how dark the grey shirt is; the darker the shirt, the greater the temperature difference.
um grey is closer to black than white so i assume white is cooler....
White shirts, much cooler. As long as they're not too tight. White reflects light better than darker colors, which absorb heat.
about 30 degrees in direct sunlight.source-http://www.t-shirtforums.com/general-t-shirt-selling-discussion/t26712.html
You'd also feel cooler in a natural fabric, cotton or linen.