YIQ Colour Model:

While an RGB monitor requires separate signals for the red, green, and blue components of an image, a television monitor utilize a single composite signal. The National Television System Committee (NTSC) colour model for forming the composite video signal is the YIQ model.

In the YIQ colour model, luminance (brightness) information is contained in the Y parameter, whereas chromaticity information (hue and purity) is incorporated into I and Q parameters. A combination of red, green, and blue intensities is chosen for the Y parameter to yield the standard luminosity curve. As Y contains the luminance information, black-and-white television monitors utilize only the Y signal. The biggest bandwidth in the NTSC video signal (around 4 MHz) is assigned to the Y information. Parameter I contain orange-cyan hue information that provides the flesh-tone shading, and occupies a bandwidth of approximately 1.5 MHz. Parameter Q carries green-magenta hue information in a bandwidth of around 0.6 MHz.