The standards you'll meet
A gamut is drawn as a triangle on a chromaticity chart, with one corner for each of the red, green and blue primaries. The corners are given as x and y coordinates. The standards a monitor spec sheet mentions are:
- sRGB: the baseline for the web and most monitors, defined by HP and Microsoft in 1996. Its primaries match Rec. 709 (HD video). Red is at (0.640, 0.330), green at (0.300, 0.600) and blue at (0.150, 0.060).
- DCI-P3: made for digital cinema. Red (0.680, 0.320), green (0.265, 0.690), blue (0.150, 0.060). It has a white point near 6300 K and a gamma of 2.6.
- Display P3: Apple's variant. It uses the DCI-P3 primaries with a D65 white point and the sRGB curve.
- Adobe RGB: made by Adobe in 1998 for print work. It keeps sRGB's red and blue and moves green out to (0.210, 0.710).
- Rec. 2020: the wide gamut for UHD and HDR video. Red (0.708, 0.292), green (0.170, 0.797), blue (0.131, 0.046), which are pure lights at 630, 532 and 467 nm.
A wider triangle holds more saturated colors.
Coverage and volume
Spec sheets quote coverage, a percentage of one of those triangles. It's a useful number with two blind spots.
First, it says how much of a standard the screen reaches, not how far past it the screen goes. A panel can cover all of sRGB and still reach much further, and the figure doesn't show that. A number over 100%, like 135% sRGB, isn't coverage at all. It's area, the size of the screen's triangle next to the standard's. Second, it's measured on a flat chart, so it ignores brightness. A real screen has a color volume, because every color also has a brightness, and a color that exists at low brightness may be out of reach at high brightness.
Wide gamut with ordinary content
Most web pages, apps and games are made for sRGB. If those values go straight to a wider panel, they get stretched to the panel's own primaries, and colors look more saturated than the maker intended. Apps that do color management convert the values correctly. For everything else, the monitor's sRGB mode holds the panel to the narrower gamut.
This is also why one hex code can look different on two screens. The Color Sync tool compares them side by side.
Gamut in HDR and on the web
HDR10 video uses Rec. 2020 primaries as its container, but content is commonly mastered on P3 displays with a D65 white point. In practice, how well a screen covers P3 matters more than a Rec. 2020 claim.
In CSS, the color-gamut media feature lets a page ask what the screen can show. The values srgb, p3 and rec2020 are thresholds, and each one matches a screen that covers that gamut or a wider one.