This surprisingly fun game popped up on the socials a couple of weeks ago. The concept, as described by its creator, Keith Cirkel:

You see two colours. Click on the line between them. That’s it. It starts easy. It does not stay easy.

Each round the colours get closer together until we find your Just Noticeable Difference - the smallest colour change you can actually see. It takes about 40 rounds. Most people land around 0.02.

My first attempt, on an iPhone, was acceptable—0.0123—especially for someone who’s colorblind. My second attempt—using a computer, with the screen brightness turned up, and bobbing and weaving my head around to find the color edge, was notably better: 0.0046.

Naturally, the color perception of the individual, the lighting conditions, and the screen used have as much of an impact as the color delta itself.

The science explaining all of this and was equally fascinating (and I managed to follow a good chunk of it!).