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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 the RGB color model 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 two-color Technicolor and other early 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 a similar color space called RGK which also has a black channel. Outside of a few low-cost high-volume applications, such as packaging and labelling, RG and RGK are no longer in use because devices providing larger gamuts such as RGB and CMYK are in widespread use. Until recently, its primary use was in low-cost light-emitting diode displays, where red and green tended to be far more common than the still nascent blue LED technology, though full-color LEDs with blue have become more common in recent years.
ColorCode 3D, a stereoscopic color scheme, uses the RG color space to simulate a broad spectrum of color in one eye; the blue portion of the spectrum transmits a black-and-white (black-and-blue) image to the other eye to give depth perception.
Color Graphics Adapter (CGA) 
The first color capable video card for the IBM PC family was 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 card as a whole implemented the "digital RGBI" 16-color space (i.e. each primary color (red, green, and blue) could be either on or off for a given pixel, and an additional intensity bit would brighten all three primaries if it was turned on for a pixel.
The color mode uses two bits to store red and green 1-bit components for each pixel (that is, colors in the RG color space) while the blue and intensity components were fixed for the entire screen.
This gave four possibilities for each single pixel: background (any one of the 16 colors the system offered, black or blue was most used; however only one background color could be chosen for the entire screen), red, green and yellow, with two possibilities of intensity selectable for the entire screen (except the background): low (darker) and high (lighter). This was known as Fixed palette #2. The Fixed palette #1 adds the blue component to all colors except the background, giving background (usually black was chosen), magenta (red+blue), cyan (green+blue) and white (yellow+blue), with two possible intensities, too.
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