The correlated color temperature, or CCT, measures the shade of whiteness that a light source has. Most of the time, incandescent lighting is going to be around 2700 K, which provides a light that has a yellow-white appearance. You will find that fluorescent tubes are able to have different CCTs so that they can fit in well with the incandescent lamps of the home, as well as other CCTs that might work better with the office, the basement, or another area. For example, the cool white fluorescents have a CCT of 4100 and are good for office lighting. This ties in quite closely with the color rendering.