What is Color Gamut?

Color gamut is defined as the range of colors which a particular device can produce or record. It is usually shown by an enclosed area of the primary colors of the device on the chromaticity diagram. For example, the primary colors of monitors are red, green, and blue. Hence, the color gamut of a monitor is shown in a triangular area enclosed by the color coordinates of the monitor's red, green, and blue colors. A chromaticity diagram with different color gamuts is shown in Figure 1.

One very important aspect of color gamuts is that they're three dimensional, not two dimensional. When we describe a color, it usually consists of three components: hue, saturation, and brightness. Since color gamut is a collection of colors, it is logical that it can also be described using hue, saturation, and brightness. So when we plot a color gamut, it should be a three dimensional graph as shown in Figure 2. However, plotting a three dimensional graph is never an easy job, even nowadays. As a result, we often use a two dimensional graph to represent a color gamut, as shown in Figure 1. The missing dimension is brightness, which is perpendicular to the screen or sheet of paper.

The most commonly known color gamut might be “NTSC” (established in 1953 by the US FCC), however, it is not a standard color gamut. “sRGB” is the standard color gamut established by the IEC in 1999. The reason for standardizing a color gamut was to promote an easier way for color reproduction. If all color-utilizing devices, including digital cameras, scanners, monitors, printers, and projectors, can reproduce the sRGB color gamut correctly, then images can be reproduced very consistently across devices. This phenomenon is due to devices having the same capability of recording (with proper calibration of course), displaying, or printing the same range of colors, so for the same image, the variation of color reproduced on different devices will be limited. Hence, color fidelity or color management could be achieved. This is also known as the concept of reproducing in-gamut colors.

However, in order to achieve color fidelity or color management, the exact RGB color coordinates on the chromaticity diagram must be agreed upon. Each pair of color coordinates specifies a distinct or unique color. For example, (x, y) = (0.64, 0.30) is not equal to (x, y) = (0.63, 0.29), even though the numbers are very close to each other. In the display industry, there is also another piece of misleading information. Some panel manufactures or display brands market their monitors as “72% NTSC” in terms of color gamut, claiming to be “sRGB compatible”. Unfortunately, this is not the case. If we take the color coordinates from each color gamut from Table 1 and calculate the area of each color gamut, we can find that the area ratio of sRGB to NTSC is 0.72 to 1. This is where the misleading concept of 72% NTSC equals to sRGB came from. The correct interpretation is “the number of colors that sRGB can reproduce is the same as 72% of NTSC, but the colors being reproduced are not necessarily the same colors.” Because the only constraint on 72% is the area ratio, and there is no information about the RGB color coordinates. Hence, we cannot say 72% NTSC color gamut is equal to sRGB color gamut. Therefore, it only makes sense when we talk about “coverage rate” when comparing two color gamuts. An example can be seen in Figure 3. On the left-hand side, both color gamuts have the same area ratio, but they do not 100% cover each other. On the right-hand side, only a certain amount of the monitor color gamut covers sRGB, and it's not 100%. It is clear that this particular monitor cannot reproduce 100% of the sRGB color gamut.