The quality of how colours are reproduced is called colour rendering. Line spectra have very good colour rendering. Line spectra allow only a single colour to be perceived well. Multi-line spectra render several colours of the respective spectrum well, but colour rendering is weaker in the intermediate ranges. Blue and green colours appear comparatively grey and dull in warm white incandescent light despite excellent colour rendering. However, these colour tones appear clear and luminous in the light of daylight white fluorescent lamps – despite poorer colour rendering. With the rendering of yellow and red colour tones, this phenomenon of weakening or intensifying of the colour effect is reversed.
Because the eye is able to adapt to light of the most different colour temperatures, the colour rendition must be determined dependent on the colour temperature. Tungsten halogen lamps feature very good colour rendition. The rendition quality of fluorescent lamps and metal halide lamps ranges from good to average. The degree of colour distortion against a reference light source is indicated using the colour rendition index Ra or the colour rendition grading system. The colour rendition index is only used for white colours of light.