What is colour blindness actually like?
Why so many people do not know they have it
Because you learn the names of colours from other people, and consistency is enough to pass. Someone who has always called a particular shade green will keep calling it green, and nothing in daily life contradicts it. It is often discovered in adulthood, and often by accident.
The design mistake it exposes
Using colour as the only signal. Red and green as the two states of a status light, or as the only distinction between lines on a chart, is unreadable for a significant minority of viewers — and also for anyone on a bad screen or in bright sun.
The fix is cheap: add a shape, a label, or a difference in brightness alongside the colour. Anything that survives a black-and-white printout works.
Whether it can be corrected
Not really. The tinted glasses sold for this increase contrast between confusable colours, which some people find genuinely useful, but they do not restore missing perception and the effect varies a lot between individuals. Phone filters do something similar on a screen.
What helps most in practice is labelling — and being told when colour actually matters.
Our iOS app for exactly this. Runs on your device — no account, no tracking.