Medium-surface temperature of black, gray, and white plug sheets was measured with thermocouples and an infrared camera. During the night, there were no medium-surface temperature differences between the plug flats; however, medium-surface temperature was 2 to 3 °C below air temperature. Medium-surface temperature increased as solar radiation (280 to 3000 nm) increased. About 80 W of solar radiation/m2 was incident on the plug-flat surface before medium-surface temperature equaled air temperature. Medium-surface temperature in the black, gray, and white flats was 6.3, 6.1, and 5.3 °C above air temperature, respectively, when 300 W of solar radiation/m2 (30% of the maximum solar radiation during the summer) was incident on the medium surface. Thus, incident solar radiation has a greater effect on medium surface temperature than plug-flat color.