The CMY color space describes colors in terms of the subtractive
primaries: cyan, magenta, and yellow. CMY is used mainly for hardcopy
devices such as color printers. Generally, the conversion from
RGB to CMY follows the equation [30]:

CMY conversion may be performed using the color matrix or a scale and bias operation. The conversion is equivalent to a scale by and a bias by . Using the 4x4 color matrix, the equation may be restated as:

Here, the incoming alpha channel must be equal to 1. If the source is RGB, the 1 will be added automatically in the format conversion stage of the pipeline.

A related color space, CMYK, uses a fourth channel (K) to represent black. Since conversion to CMYK requires a operation, it cannot be performed using the color matrix.

The extension `EXT_CMYKA` also supports conversion to and from CMYK
and CMYKA. This extension is currently supported by Evans
& Sutherland.