What is being varied, and what is not
Every tile in a grid shares a hue and a saturation. The odd tile differs only in lightness, and the gap shrinks as your score rises — from around twenty points on a hundred-point lightness scale at the start, down to a floor of three. Hue is deliberately left alone. Varying hue would make the difficulty depend on which part of the spectrum a given round happened to land in, and that varies enormously between people, which is the opposite of what a score is supposed to mean.
Lightness discrimination is also the more robust thing to test on an uncontrolled screen. It survives a mediocre panel better than hue does, though as the next section explains, better is not the same as well.
Why this cannot say anything about colour vision
Colour vision deficiency is a specific condition involving the cone photoreceptors, and it affects the discrimination of particular hue pairs — most commonly red against green. Diagnosing it means presenting standardised plates or arrangement tests under specified illumination, on printed materials whose colours are known, and interpreting the pattern of errors. Every one of those requirements is absent here.
Your display has an unknown colour profile, an unknown brightness and possibly a night-shift filter warming everything. The ambient light in your room changes the apparent contrast of what is on screen. And the task is a lightness task, which a person with a red-green deficiency may well do perfectly. A high score rules nothing out and a low score indicates nothing. If colour causes you real trouble — matching clothes, reading a chart, a job that requires a colour test — that is a question for an optometrist or ophthalmologist, and there is no substitute for one.
Things that move the score that have nothing to do with your eyes
Screen brightness is the big one. Very dim screens crush the difference between dark shades, and a low-brightness run feels dramatically harder. Night-shift and blue-light filters shift the whole image and cost most people several points. A glossy panel with a window behind you loses low-contrast detail to reflections. An OLED and an older LCD will not render the same lightness step the same way.
Where you look matters too. Peripheral vision is often better at catching a low-contrast oddity than the fovea is — the odd tile pops out more when you soften your gaze at the middle of the grid than when you scan tile by tile. Scanning systematically feels more thorough and is usually slower. If you want two runs to be comparable, hold the screen brightness, the room lighting and any filter setting constant between them, and treat everything else as noise.
Questions people ask
Is this a colour blindness test?
No, and it should not be used as one. It varies lightness rather than hue, on a display with unknown colour characteristics, in unknown lighting. Colour vision deficiency is diagnosed with standardised plates or arrangement tests under controlled illumination by a clinician. If you have any real concern, book an eye test — this page is not a substitute for one in any way.
Do wrong clicks cost me points?
Not directly. The tile fades so you can see you already tried it, the miss count goes up, and nothing is subtracted. The real cost is the time, which in a timed run amounts to the same thing.
Why does it get harder in two ways at once?
The grid grows and the lightness gap shrinks together, so later rounds have both more tiles to search and a smaller difference to find. That keeps the score climbing at a roughly steady rate rather than running away, but it also means a score of twenty was not twice as hard as ten — it was considerably more than twice.
Should I turn off night mode before playing?
If you want to compare against your own earlier runs, keep the setting the same in both, whatever it is. If you simply want the easiest conditions, a colour filter off with normal room lighting will read a few points higher than a warm dim screen.