The color temperature of a light source is the temperature of an ideal black-body radiator that radiates light of a color comparable to that of the light source. In practice, color temperature is meaningful only for light sources that do in fact correspond somewhat closely to the radiation of some black body, i.e., light in a range going from red to orange to yellow to white to blueish white. Color temperature is conventionally expressed in kelvins, using the symbol K, a unit of measure for absolute temperature.
From:See the comments section of this LightNowBlog.com article Archived 2017-03-07 at the Wayback Machine on the recommendations of the American Medical Association to prefer LED-lighting with cooler color temperatures (i.e. warmer color).