How to Use This Tool
Paste your palette. Each colour is shown as it appears with the three main forms of colour vision deficiency, and any pairs that collapse into each other are listed.
What the three types are
- Deuteranopia — the green cone is missing. The most common form, and the one worth checking first.
- Protanopia — the red cone is missing. Similar effect, with red also appearing darker than it does to others.
- Tritanopia — the blue cone is missing. Rare, and it breaks blue-yellow distinctions rather than red-green.
Roughly 8% of men and 0.5% of women have some red-green deficiency. In a room of a hundred people that is around four, and in a large user base it is a permanent fraction you will never hear from, because the interface does not appear broken to them — it appears ambiguous.
The convention that causes it
Red for bad and green for good is the most widespread signalling pattern in software, and red and green are precisely the pair that collapses. A build status column, a stock ticker, a form validation state, a heatmap — all of them commonly encode the entire meaning in the one distinction that a twelfth of male readers cannot make.
The fix is not to abandon the colours. It is to make sure colour is never the only carrier of the message.
What to do instead
- Add a shape. A tick and a cross carry the meaning with no colour at all, and cost almost nothing to add.
- Add a word. "Failed" next to the red dot removes the ambiguity entirely.
- Vary lightness, not just hue. Two colours that differ in lightness stay distinguishable under every form of deficiency, and in greyscale.
- Prefer blue and orange to red and green for two-way distinctions. That pair survives red-green deficiency intact and is why so many accessible palettes look the way they do.
- Do not rely on a legend that maps colours to meanings if the colours are the only difference — the legend has the same problem as the chart.
What this simulation is and is not
It uses the standard transform into cone-response space used in the research literature, which is a reasonable model of dichromacy — the complete absence of one cone type. Most people with a deficiency have anomalous trichromacy instead, where a cone is shifted rather than missing, and their experience sits somewhere between this simulation and typical vision.
So treat a collapsed pair here as a genuine warning and an intact pair as reassurance rather than proof. The reliable test is whether the information survives with the colour removed entirely: if you convert your interface to greyscale and it still makes sense, you are fine.
