In an (M) NTSC or (B, D, G, H, I) PAL video signal, the luma (black and white) and the chroma (color) information are combined together. To decode an NTSC or PAL video signal, the luma and chroma must be separated. The chroma bandpass filter removes the luma from the video signal, leaving the chroma relatively intact. This works fairly well except in certain images where the luma information and chroma information overlap, meaning chroma and luminance information occupy the same frequency space. Depending on the filtering technique used, it can be difficult for the filter to separate the chroma from the luminance information.

This results in some luminance information being interpreted as chroma and some chroma information being interpreted as luminance. The effects of this improper separation of luminance and chroma are especially noticeable when the television scene contains objects with thin, closely spaced black and white lines. As the camera moves across this object, a rainbow of colors will appear in the object indicating the improper separation of the luminance and chroma information.