Depending on the light source, the color of the reflected light is composed by very different spectres.
Pure temperature beamers like for example bulbs (artificial light), the sun or candles have a continuous color spectrum which is being shifted between blue and yellow according to the temperature of the beamer. This is why it is defined as color temperature.
Light sources like gas discharge lamps do not "produce" the light energy from thermal energy but take it from other physical processes. Therefore, they do not have a continuous color spectrum but a line spectrum. The consequence is that there is also a shift to the magenta-green axis, apart from the blue-yellow shift. This shift may only be detected by means of a third measuring cell.