Your Default Chart Palette Fails for 1 in 12 Men
Most charting libraries pick their default colours for hue variety. Hue is exactly what red-green colour blindness takes away, which is why the defaults fall apart.
You ship a dashboard with five series on it. It looks fine. A week later someone mentions, almost apologetically, that they cannot tell two of the lines apart — and they have been guessing which is which since the day it launched.
The numbers are worse than they feel
Roughly 8% of men of Northern European descent have some form of red-green colour vision deficiency, against about 0.5% of women. In a team of thirty engineers, that is usually one or two people. In a public dashboard with ten thousand monthly readers, it is several hundred.
The most common form is deuteranomaly — reduced rather than absent green sensitivity. That matters, because it means most affected readers do not experience the world in greyscale. They see colour, just with one axis compressed, which is precisely why the problem goes unreported: the chart does not look broken to them, it looks ambiguous. And ambiguous feels like their fault.
Default palettes fail for a structural reason
Chart libraries pick their default colours to be maximally distinct in hue. That is a reasonable objective, and it is also the exact axis that red-green deficiency compresses. A palette optimised purely for hue variety is optimised for the one property some of your readers cannot use.
Lightness survives. Saturation partly survives. Hue does not. So a palette that varies only in hue degrades to a set of near-identical mid-tones, while a palette that also varies in lightness stays readable even in full greyscale.
Check your own palette
The default loaded below is a typical library palette — the sort of thing you get for free and never think about again. Switch the colour vision selector to deuteranopia and watch what happens to the red and the green.
Your palette
Pairs that become hard to tell apart
Every colour stays distinguishable.
The dashed outlines mark colours that have moved within ΔE 20 of another colour in the palette — close enough that most readers will stop being able to separate them reliably in a chart, where the patches are small and rarely adjacent.
What actually fixes it
Three things, in descending order of how much they buy you:
- Spread the palette across lightness, not just hue. If your colours are still separable when you convert the chart to greyscale, they will survive every deficiency, because lightness is the one channel nobody loses.
- Encode the same information twice. Line style, marker shape, or a direct label at the end of each line means colour is a convenience rather than the only key. This is the single most robust fix, and it also helps on a projector, on a photocopy, and in bright sunlight.
- Start from a tested palette. The Okabe–Ito set is eight saturated colours designed to stay separable under every common deficiency; ColorBrewer’s qualitative sets are the other well-worn option. Neither is dull, which is a useful counter to the assumption that accessible means washed out.
What does not work is a rule like “never use red and green together.” The deficiency collapses an entire axis, so orange and green collide, pink and grey collide, teal and grey collide. There are more failure modes than a rule of thumb can hold. Simulating is cheaper than memorising.
Common follow-ups
How common is colour blindness, really?
Roughly 8% of men and 0.5% of women of Northern European descent have some form of red-green colour vision deficiency. Deuteranomaly — reduced green sensitivity — is by far the most common. Blue-yellow deficiency is rarer, on the order of 1 in 10,000.
Is it enough to just avoid red and green together?
No. The problem is not the two names, it is that red-green deficiency collapses a whole axis of colour space. Orange and green, pink and grey, and teal and grey can all collide too. That is why simulating is more reliable than following rules of thumb.
Does a colour-blind-safe palette have to look dull?
No, but it does have to spend its variation on lightness as well as hue. The Okabe–Ito palette is eight saturated colours that stay separable under every common deficiency, which is a good demonstration that the trade-off is smaller than people assume.