Subpixel

On most screens there is no such thing as a white pixel, only red, green and blue elements lit at the same time.¹

The exception is the large OLED television, many of which carry a fourth subpixel that is white.^{1, 2}

A liquid crystal display does not emit light itself.¹ It modulates light from a backlight, and its colours come from an array of colour filters in front of the liquid crystal layer.¹ One 2250 pixel-per-inch Samsung panel has a pixel pitch of 11.28 micrometres and a subpixel pitch of 3.76, so each pixel is exactly three subpixels wide.¹

Phone screens based on OLED work differently, because each subpixel is its own light emitter in red, green or blue.¹ Most of them use an arrangement called PenTile, in which the subpixels are not laid out in equal stripes.¹ The blue element is made larger, so it can run at a lower current and last longer.¹

Why three coloured lights should look white was worked out before any of this hardware existed.³ In 1801 the physician Thomas Young reasoned that colour vision could be explained by just three kinds of receptor in the eye, each sensitive to a different part of the spectrum.³ He published the idea in 1802.³ Pirenne, writing in Nature in 1944, judged that the strength of Young's argument remained undiminished.³

Large OLED televisions break the pattern.¹ Instead of separate red, green and blue emitters, they use a white OLED layer behind colour filters.¹ F. Chesterman and colleagues measured one such 55-inch panel in 2016, describing it as a WRGB display with four subpixels per pixel: white, red, green and blue.² Their power model had to account for how much each of the four contributed to the screen's white.²

On most screens, then, the white of a blank page is a mixture made in the viewer's head. The pixels supply only the ingredients.