Cyan is the red channel test
Cyan is green and blue at full power with red switched off. Every pixel in this frame should be emitting nothing at all from its red subpixel, so a pixel that shows as white, pink or pale against the cyan has a red subpixel stuck on. Stuck red is one of the more common single subpixel faults, and it is nearly invisible on ordinary content because most photographs contain plenty of red.
Why testers use cyan, magenta and yellow as well as the primaries
Red, green and blue between them find dead subpixels: a black dot on a primary means that element is not lighting. Cyan, magenta and yellow find the opposite fault, a subpixel stuck permanently on, because each of them switches one channel completely off and leaves nowhere for a stuck element to hide. Six screens, two failure modes, complete coverage. The dead pixel test on this site runs all of them in sequence so you do not have to remember the order.
Cyan as a working surface
- Cyan is a common alternative chroma key colour when a subject is wearing green, and a tablet showing full screen cyan works for a small subject in the same way a green one does.
- It is a good backdrop for photographing warm coloured objects, which separate cleanly from a cool field.
- On a calibrated display, a flat cyan should look neither greenish nor blueish. A clear bias one way is a white balance problem worth chasing in the monitor settings.