Answer
The coolest stars appear reddish. Medium-temperature stars look whitish. Even hotter stars appear bluish.
Work Step by Step
Objects at any temperature greater than absolute zero will emit radiation. The peak frequency of this radiation is proportional to the object's absolute temperature - the hotter the object, the higher the frequency of the peak radiation.
Hotter stars emit more radiation at all frequencies, and the frequency peak of that radiation moves upward.
Red is the color with the lowest frequency (and the longest wavelength), so a infrared or red peak frequency corresponds to the coolest stars.
As the peak moves through the middle of the visible spectrum, the star appears white - a roughly equal amount of light is emerging at each visible frequency.
Even hotter stars emit radiation with a peak frequency in the ultraviolet or at the high-frequency end of the visible spectrum, which is blue-violet.
