TV Color Gamut Explained: The Signal Is Not the Screen

What is color gamut on a TV?

A TV's color gamut is the range of colors its screen can physically reproduce, usually drawn as a triangle whose corners are the panel's purest red, green and blue. That is only one of several gamuts in play: the content has its own color space, the signal arrives in a labelled container, and the TV maps between them. "Wide color gamut" means a range larger than the HD standard, Rec. 709, not a guarantee of accurate color.

Konica Minolta, a maker of display measuring instruments, puts it plainly in its guide to evaluating display color gamut: "a given display can only reproduce the colors that lie inside the color triangle formed by its primary colors." The rest of this guide is about the gap between that triangle and the ones the picture was made for.

Why does one picture involve several color spaces?

A single shot of a red coat travels through at least six distinct ideas before it reaches your eyes, and spec sheets tend to blur them into one number:

Keeping these apart is the whole trick. A TV can accept a wide-gamut signal yet show a narrower range, and a setting can make colors look "wider" by stretching them rather than revealing anything new.

What do Rec. 709, P3 and Rec. 2020 actually define?

Each standard fixes three primaries and a white point as coordinates on the CIE 1931 chromaticity diagram, a map of every hue and saturation the eye can see. The triangle between the three primaries is the gamut.

The Rec. 709 row comes from the HDTV standard ITU-R BT.709-6 and the Rec. 2020 row from the UHDTV standard ITU-R BT.2020-2. The HDR standard ITU-R BT.2100-3 uses the same primaries, describing them as monochromatic 630 nm red, 532 nm green and 467 nm blue. The P3 row is "display-p3" from the W3C's CSS Color Module Level 4, which "uses the same primary chromaticities as DCI-P3, but with a D65 whitepoint."

Read across the table and a pattern appears. P3 pushes red and green outward from Rec. 709 but shares its blue corner exactly. Rec. 2020 pushes all three corners further out than Rec. 709, and its green moves the furthest. All three use the same D65 white, the daylight-like white that color temperature settings aim at.

What does "wide color gamut" mean on a TV?

Wide color gamut (WCG) describes a display or signal that reaches beyond Rec. 709. BT.2020 itself lists "wider colour gamut" among the improvements expected of ultra-high-definition television, alongside wider dynamic range.

What it does not mean is "shows all of Rec. 2020." The W3C's CSS Color specification is blunt on this point: the Rec. 2020 primaries "are physically realizable, but with difficulty as they lie very close to the spectral locus", and "current displays are unable to reproduce the full gamut of rec2020."

That is one reason TVs are commonly judged against P3 rather than Rec. 2020. When the UHD Alliance announced its Ultra HD Premium criteria in January 2016, it asked displays to accept "BT.2020 color representation" as a signal input while reproducing "More than 90% of P3 colors", and it recommended mastering displays reach a "Minimum 100% of P3 colors." The signal, the screen and the mastering monitor each had a different color target in one document.

What does "mastered in P3, delivered in Rec. 2020" mean?

The signal container is the color space a video is labelled as. The mastering gamut is the range of colors the grading monitor could actually display while the colorist approved the look. They are often not the same, and that is normal.

Think of a large shipping box holding a smaller parcel. Plotted from the table above, the Rec. 2020 triangle encloses nearly all of the P3 triangle (only a hair-thin sliver near P3's red corner falls outside), so a film graded on a P3 monitor can travel in a Rec. 2020 container essentially intact. Its colors simply never reach the outer corners of the box.

The labelling matters, because downstream equipment trusts it. YouTube's guidance on uploading HDR video warns that "a common mistake is to master in P3, then tag the result using Rec. 2020 primaries," and that this "will result in an oversaturated look with shifted hues." The problem there is not the large container; it is P3 numbers being read as though they were Rec. 2020 numbers.

HDR video can also carry mastering display metadata. The SMPTE standard ST 2086 specifies metadata for "the color volume (the color primaries, white point, and luminance range) of the display that was used in mastering video content." The metadata is optional and describes the mastering display; it does not by itself guarantee your TV will match it.

Why can't one percentage tell you how good the color is?

A gamut percentage compresses a three-dimensional question into one flat number, and several choices hide inside it.

First, area versus coverage. Konica Minolta's guide explains that the chromaticity area ratio (CAR) compares the size of the display's triangle with a standard's, and warns that "using CAR can be misleading when color coverage is concerned." A display triangle can match a standard's area while sitting in a different place, missing some of the standard's colors and adding others no content asks for. Coverage, which counts only the overlap, is the figure the same guide calls "easier to understand and is a more accurate specification."

Second, which diagram. Gamut figures are drawn either on the CIE 1931 xy diagram or on the CIE 1976 u′v′ uniform-chromaticity-scale diagram, and Konica Minolta notes that the area ratio "will also depend on the chromaticity chart used." The same panel can therefore earn two different area figures against one standard, so figures that do not name their diagram cannot be fairly compared.

Third, brightness. A triangle has no height, whereas ST 2086 defines color volume as primaries, white point and luminance range together. In practice, a TV can reach a saturated red at modest brightness yet be unable to hold it in a bright highlight. This is where gamut and HDR become one conversation: good coverage with limited brightness can still render a red neon sign with less punch than the triangle suggests.

What happens to colors the TV cannot show?

When the signal asks for a color outside the panel's gamut, the TV has to substitute something. That substitution is gamut mapping. The ITU's Report BT.2407, which studies converting Rec. 2020 material to Rec. 709, sets out a wish list that is a fair yardstick for any TV: colors already inside the smaller gamut should be unchanged, perceived hue change should be as small as possible, and there should be no significant loss of spatial detail. It then states that "there is no universal gamut mapping method which can achieve all of these requirements simultaneously," and that "modification of colours outside the BT.709 gamut is unavoidable."

The crudest approach is hard clipping, where out-of-range values are cut to the nearest limit. The report notes this "can lead to significant hue shifts and loss of spatial detail", while adding that "most content will look just fine." Picture a sunset sky that keeps its gradations versus one flattened into a single orange slab. Gentler mappers trade a little saturation at the edges for preserved detail and hue. Mapping also runs the other way: ordinary Rec. 709 content belongs inside Rec. 709's smaller triangle, not stretched across the panel's full native gamut.

What do the color space and saturation settings do?

Many TVs have a setting named something like Color Space or Color Gamut. Samsung's support page on the Color Space setting describes the trade-off clearly: in Native mode "this will expand all incoming signals to the native color gamut of the TV", which can look more vivid but risks oversaturation, while in Auto mode "the TV will adjust the display gamut to match the incoming signal." Menu names and options vary between brands and models, so check your own manual.

The familiar Color or Saturation slider is a different tool. It scales how intense colors appear relative to white, but it cannot move the panel's primaries, so it can make a narrow-gamut screen look louder without showing a single new hue. Vivid and dynamic picture modes often combine both effects, which is why skin can look sunburned and grass radioactive on a showroom wall.

For accurate color, a sensible starting point is a cinema- or filmmaker-style mode with the gamut setting on its automatic, signal-matching option; the first picture settings guide walks through that sequence. If you prefer a bolder look, a native setting is a legitimate taste choice, as long as you know it is a stretch rather than a reveal.

How should you read gamut claims when shopping?

A few questions turn a color badge back into information:

• Which standard? A P3 figure and a Rec. 2020 figure measure against different triangles, so they are not comparable.
• Coverage or area? Coverage tells you how much of the standard the TV reproduces; area alone does not.
• Which diagram? An xy figure and a u′v′ figure for the same panel can differ.
• At what brightness? A flat triangle says nothing about how saturated colors hold up in bright HDR highlights.
• In which mode? A gamut measured in a vivid mode tells you little about the accurate one you will actually use.

Then trust your eyes on familiar material: skin tones, foliage, a blue sky, a dimly lit red. Wide color is meant to add range where the content asks for it, not to make every frame shout.

Sources

• Konica Minolta Sensing: Evaluating Color Gamut of Smartphone Display
• ITU-R Recommendation BT.709-6: Parameter values for the HDTV standards
• ITU-R Recommendation BT.2020-2: Parameter values for ultra-high definition television systems
• ITU-R Recommendation BT.2100-3: Image parameter values for high dynamic range television
• W3C: CSS Color Module Level 4
• UHD Alliance: Defines Premium Home Entertainment Experience (press release, January 2016)
• YouTube Help: Upload High Dynamic Range (HDR) videos
• SMPTE ST 2086:2018: Mastering Display Color Volume Metadata
• CIE e-ILV 17-23-073: CIE 1976 uniform-chromaticity-scale diagram
• ITU-R Report BT.2407: Colour gamut conversion from BT.2020 to BT.709
• Samsung Support: What Is The Color Space Setting?