A color space is a three-dimensional model of color.
Red, green, and blue are placed on the three axes according to the chart below.
The color space represents all of the possible colors.
But, the devices in a photography system may not be able to display or print all of the colors.
The portion that a device can use is called the color gamut.
This is the area in the triangle below.
If a color in a file is not a color within the gamut (the triangular area), then the color is called out-of-gamut.
In practice, the term color space is often substituted, incorrectly, for color gamut.
From here on, color space will be used, incorrectly, to refer to the color gamut.
There are two commonly used color spaces.