Stars of blue color have the highest surface temperature.
Scientists have been able to approximately determine the temperature of stars based on the spectrum of a black body, which absorbs all radiation. So, based on that system, they figured out that the stars with the lowest temperature are red stars, their temperature being about 3,000°C, while the hottest stars are blue with the temperature of 7,000+ °C. For reference, the Sun is a yellow star with the temperature of about 5,500°C.
