“out of the RED gamut.”

In photographic parlance Gamut refers to the complete spectrum of colors that a given color system can produce. In photography, the gamut available will depend on whether the photographer is working with the RGB (red, green, blue) or the CMYK (cyan, magenta, yellow, key/black) color model.

Because each of these color systems starts with uniquely different set of primary colors, their combinations produce a distinct set of secondary and tertiary colors. For example, while the gamut of RGB includes a pure blue, that of CMYK will render a bluish purple. Depending of the subjects of the photograph and the desired effect, a photographer will elect one of the color models to have the resulting gamut in his photos.

If a color system lacks a certain shade within its gamut, then the color is “out of the gamut.”

Keep in mind that photographs don’t have to be in color to have a gamut. For example, pictures taken with black and white film or treated with sepia each have their own unique gamut. While black and white pictures will have a color spectrum that ranges from white to gray to black, the gamut for sepia toned pictures will span from white to light brown to dark brown.

Although it originated as a photographic term, gamut has come to have a broader meaning in the English language. In general terms, gamut can refer to any kind of spectrum.

*explanation stolen from the www(photography.com) and adapted slightly