Monitor Colour Accuracy Explained – sRGB, DCI P3, DeltaE and more!

You’ve heard us reviewers talk about monitor’s colour accuracy, colour gamut coverage and DeltaE’s plenty, even things like sRGB, AdobeRGB and DCI P3 colour spaces, but often we don’t actually explain what all that means, so give me a few minutes to run you through it. At a base level, your display is generally made up of red, green and blue sub pixels. How much light each of these sub pixels lets through determines what colour you perceive. It’s an amazing trick that we use to make our eyes see a whole range – literally millions – of colours, without needing a way to actually produce more than three.

Colour accuracy is pretty simple. It’s how close your screen gets to displaying any given colour correctly. A very colour accurate display will display colours imperceptibly close to the actual colour you expect, whereas an inaccurate display will show you colours that are wildly different from what you should be seeing. The way this is measured is with a value called the “DeltaE”. If you picture the whole visible light spectrum on a graph you’d get something that looks like this. If your monitor tries to display this purple colour, but ends up leaking a little bit of green so you end up with this shade instead, that’s not accurate and therefore would have a higher DeltaE value.

DeltaE is actually a remarkably complicated calculation, and there’s actually been multiple versions of the figure. CIE, the international commission on illumination, an over century old organisation, first offered DeltaE in 1976 as just the Euclidean distance formula. That had some problems especially with saturation, and in short it was revised twice, settling in with this. DeltaE 2000. This mess is how you actually calculate DeltaE these days, so please forgive me if I don’t give you any actual numbers for my hypothetical examples. Still, the rule of thumb is a DeltaE of less than two is what you are after for an accurate-to-the-eye display. Ideally the panel would just perfectly reproduce all colours, but the real world is far from the ideal world.

So what about this whole “colour space” talk? Well if we take a look at our visible light chart here, not all of this can be displayed by monitors. There is an almost infinite number of shades you could pick from here, so the IEC (international electrotechnical commission) standardised the sRGB colour space Microsoft and HP had been using since 1996 as the beautifully named IEC 61966-2-1 standard. I’ll stick with sRGB, it’s a bit more catchy to me. sRGB is one of the smallest colour spaces, but is the standard for web content. It’s obviously made up of red, green and blue values, although generally those values are represented as an 8 bit binary byte per colour channel. That’s called 8 bits per channel, and means you have 255 different values per colour, for a total of around 16.7 million combinations.

As an aside, you might have noticed a setting for “limited dynamic range”. That’s when your graphics card will artificially clip values to RGB 16 on the low end and RGB 235 on the high end. For most monitors you want full dynamic range, but some especially older TVs won’t display RGB 0 or RGB 255 right so you’ll need the limited range setting instead.

So what about other colour spaces? Well, the AdobeRGB and DCI P3 spaces are much wider, with AdobeRGB based on all the colours a CMYK printer can output since as you’d expect people who use Adobe software like Photoshop normally want to print their work and it’d be great if they can see what colour their work is before actually printing it. DCI P3 is from DCI, the Digital Cinema Initiative, which is why it’s more common to hear about in the video editing space, and stretches more into the deeper reds and lighter greens compared to AdobeRGB’s focus on exclusively more green. Often to be able to display more than the 16.7 million colours sRGB usually offers, you need to use more bits per channel. Most colour-focused displays offer 10 bits per channel, which means they have over a billion colour combinations available to them. Quite an upgrade!

So then, how do you test for both the accuracy and the colour gamut coverage. Well that starts with a way of accurately recording what colours your display is outputting. A device like this, the Datacolor SpyderX (not sponsored by the way, this is just what I have), has a 6 axis colour sensor inside that acts a bit like a camera to capture how much red, green and blue light is emitted. Their test software then has the display show various colours to measure both the gamut and, in a separate test, the accuracy too.

You might have also heard about colour calibration. It’s a very useful tool, but it’s important to know that unless the profile is stored on the scaler itself (like some of LGs models), it’s your graphics card that is applying the calibration changes. Basically a tool like the SpyderX records how off your display is with each of the colours, then saves a calibration file to your graphics card. That means when your graphics card goes to draw a new frame, it distorts the true colours to effectively cancel out the inaccuracies of your monitor’s panel. Say your monitor constantly shows slightly too much green, well your graphics card will tone any green values down to compensate.