Dead, hot, and stuck: three different defects with one name
"Dead pixel" gets used loosely, but a panel can fail in a few distinct ways. A dark dot defect (a dead or dark pixel) is a pixel stuck fully off: the backlight is there, but no light reaches through at that spot, so it shows as a black dot against any bright background. A bright dot defect (a hot pixel) is the opposite: a sub-pixel stuck fully on, showing as a colored or white dot against a dark background regardless of what the rest of the screen displays.
A stuck sub-pixel is narrower still. Each pixel is built from three sub-pixels, red, green, and blue, and a stuck sub-pixel is just one of those three frozen on or off while the other two keep working. That's why the red, green, and blue full-screen tests exist separately: a stuck red sub-pixel is invisible on a pure-red screen (everything is red anyway) but stands out clearly on a pure green or blue one.
How the two modes in this tool work
The solid-color test cycles through six full-screen colors, black, white, red, green, blue, and 50% gray, and you click or use the arrow keys to step through them while scanning for a dot that stays put. Gray sits between black and white and often reveals uneven backlighting or clouding that pure black or white can hide.
The pixel-refresh mode instead flashes through those colors rapidly (8 times per second here) for a duration you pick. Rapid color cycling like this is a documented approach some users report success with for a stuck sub-pixel specifically, on the theory that the rapid switching can jar loose the liquid crystal material behind a sub-pixel that is stuck rather than physically dead. It has no effect on a dead or hot pixel, since those come from a failed transistor rather than a temporarily stuck crystal, and even on a stuck sub-pixel it isn't guaranteed to work.
The ISO 13406-2 class system, and why "ISO-approved" doesn't mean flawless
ISO 13406-2 was the ergonomics standard that first defined pixel fault classes for flat-panel displays, from Class I (zero defects tolerated) down to Class IV (up to 50 hot pixels, 150 dead pixels, and 500 stuck sub-pixels allowed per million pixels). Most manufacturers built their displays to Class II, the second-strictest tier. The standard itself was withdrawn in the 2000s and formally revised by the ISO 9241-302/303/305/307:2008 series, but the old Class II numbers are still the figures most commonly quoted in manufacturer support pages and consumer pixel-policy discussions today.
None of this is a legal requirement. ISO classes are guidelines a manufacturer can choose to follow, ignore, or reference loosely, and Wikipedia's own summary of the standard notes manufacturers have used it as grounds for declining to accept an otherwise-working panel as defective. If a return or exchange matters to you, the number that actually counts is whatever your specific manufacturer's current warranty page says, not the class table below.
Worked example: what "allowed" looks like on a real screen
Class II allows 2 hot pixels and 2 dead pixels per million pixels. A 1920×1080 monitor has 1,920 × 1,080 = 2,073,600 pixels, so the allowance scales to 2 × 2,073,600 ÷ 1,000,000 = 4.15, rounded down to 4 hot pixels and 4 dead pixels before the panel would fall outside Class II. The stuck-sub-pixel allowance is looser, 5 per million, which works out to 5 × 2,073,600 ÷ 1,000,000 = 10.37, rounded down to 10 on the same panel. Move up to a 3840×2160 (4K) panel with 8,294,400 pixels and the same 2-per-million hot/dead allowance scales to 16, since a 4K panel simply has four times as many pixels for defects to be spread across.
Before you assume it's a defect
Wipe the screen with a dry microfiber cloth first. A speck of dust or a smudge can look identical to a dead pixel from a normal viewing distance, and it disappears with a wipe while a pixel defect does not. If it survives a cleaning and shows up in the same screen location across every color in the test, at every viewing angle, it is a pixel-level defect, not something sitting on the glass.