Until this year, all colour TVs created pictures by mixing red, green and blue light. Every single pixel on colour-TV screen has a red, green and blue sub-pixel. For red objects, the TV just lights up red pixels, ditto for blue and green.

What about the gazillions of other colours found in nature? TVs portray them by blending red, green and blue light. Mix red and blue and you've got magenta, plus various shades of purple. Mix blue and green, and you've got cyan, plus turquoise, aquamarine and other hues. To make yellow, the TV mixes light from red and green sub-pixels (yes, that is how additive colours work); tweaking the mix will produce orange, yellow-green and other shades. Depending on the program content and precision of the TV's processor, it's possible to produce a billion or so colours by mixing red, green and blue.

However, there are colours beyond the range of conventional RGB displays: some shades of gold, and the more intense hues of green, turquoise and aquamarine. Extending the range of colours that we can bring into our home theatres is the rationale behind Sharp's new Quattron flat panels. These add yellow sub-pixels to the customary mixture of red, green and blue. Whereas conventional flat panels can produce a billion colours, Sharp says its new four-colour displays can produce a trillion. The four-colour system also enables the TV to produce more subtle gradations of tone and colours that are inside or outside the standard range (or gamut) for TV, Sharp adds.

Much of the demo material Sharp used to show off its Quattron TVs during the January launch was rich in golds and yellows: a field of sunflowers, and an extended close-up of a saxophone full of metallic golds and rich browns. Not surprisingly for a TV with two million yellow sub-pixels, the Quattron display conveyed these shades more convincingly than one of Sharp's conventional LCDs. The Quattron display also did a better job with the rich blues and greens in a clip of a Caribbean beach - a bit of a surprise until you consider that these are exactly the shades that lie outside the standard range of RGB colours.

Whether they're showing off cameras, printers, TVs or loudspeakers, manufacturers always use demonstration material that will make their products look their best. But these demonstrations don't always translate in home use, when people are using their own content. How, I wondered, would these TVs look with other program content: movies, sports, documentaries and prime-time TV?

It's not an academic question, because the way Sharp's Quattron TVs produce colour pictures is different from the way they're recorded. All colour TV programs - broadcast, cable, satellite, DVD and Blu-ray - are stored and transmitted as mixtures of red, green and blue. Basically, RGB displays create pictures by responding directly to the levels of red, green and blue in the video signal.

A four-colour display like one of Sharp's Quattron models has to improvise. On a moment-by-moment and pixel-by-pixel basis, the TV's processor has to analyze the program content, and determine how to produce three-colour video on a four-colour screen. It has to decide when to add yellow light to the mixture (because that information is not in the signal) and how to adjust red, green and blue to compensate for the added yellow. I have seen displays that purported to produce colours beyond the standard range for TV. With some content, the effect was very impressive; with other programs, it was artificial.

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