The YIQ colour space model is use in U.S. commercial colour television broadcasting (NTSC). It is a rotation of the RGB colour space such that the Y axis contains the luminance information, allowing backwards-compatibility with black-and-white colour Tv's, which display only this axis of the colour space. The chrominance information is contained in the I (orange-blue) and Q (purple-green) axes, which are roughly orthogonal. The reason for this arrangement is that the human visual system is much more sensitive to changes in the I axis than in the Q axis, allowing the Q axis to be transmitted with less fidelity, conserving bandwidth.
The television broadcasting model PAL, used in the UK, most of Europe, and some other places such as Hong Kong, uses a closely related colour space called YUV: the difference between the two is a 33 degree rotation of the chrominance axes.
The exact scaling of the chrominance axes in these colour spaces was defined to fit the emission spectra of the phosphors used in colour Tv screens in various parts of the world. These change over time as technology improves, making these colour spaces subject to change in terms of the conversion to and from RGB. In addition, the axes are not exactly orthogonal. Therefore, in general these colour spaces have no place in digital image processing.