RG color space
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The RG or red-green color space is a color space that uses only two colors, red and green. It is an additive format, similar to RGB color space but without a blue channel. Thus, blue is said to be out of gamut. This format is not in use today, and was only used on the earliest color processes for films; by comparison to a full spectrum, its poor color reproduction made it undesirable. The system cannot create white naturally, and many colors are distorted.
Any color containing a blue color component can't be replicated accurately in the RG color space. There is also a similar color space called RGK which also has a black channel; it also is no longer in use. Systems providing larger gamuts include RGB, CMYK, and various other colorspaces.
[edit] Color Graphics Adapter (CGA)
The first color capable cards and monitors for IBM PC family where the Color Graphics Adapter (CGA), which includes two graphic modes: 320×200 pixels with four colors (two bits per pixel) and 640×200 pixels black-and-white (one bit per pixel). The color mode use two bits to store red and green 1-bit components (that is, colors in the RG color space) to obtain four combinations: black, red, green and yellow, with two possibilities of intensity: low (darker) and high (lighter). This was known as Fixed palette #2. The Fixed palette #1 adds the blue component, giving black, magenta (red+blue), cyan (green+blue) and white (yellow+blue), with two possible intensities, too.
