The RG color space can produce shades of red, green, and yellow.

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 Technicolor films; its poor color reproduction made it undesirable. The system cannot create white, and many colors are distorted.

In the RG color space, magenta, which contains a blue component, becomes orange; shades of blue appear muddy brown or black.

Any color containing a blue color component can't be replicated accurately in the RG color space. A few years later, Technicolor created a 3-color system, which is now RGB. There is also a similar color space called RGK which also has a key channel; it also is no longer in use. Systems providing larger gamuts include RGB, CMYK, and various other colorspaces.