How color-blind people see, and how far to trust a simulator

What it is like day to day

The National Eye Institute describes each type by the colors it mixes up. The mild red-green types, deuteranomaly and protanomaly, make some greens look redder or some reds look greener and dimmer, and it says they usually do not get in the way of normal activities. Only with complete color vision deficiency does a person see no color at all.

For the full red-green types, protanopia and deuteranopia, the NEI says they make someone “unable to tell the difference between red and green at all.” That is about the hue. Many reds and greens still differ in how light or dark they are, so some pairs can still be told apart and others merge completely. The designer part below shows both kinds.

The blue-yellow group is described a little differently on the NEI's two pages. Its overview page says this type “makes blue and yellow look the same.” Its page on the types lists the pairs: the milder tritanomaly “makes it hard to tell the difference between blue and green and between yellow and red”, and tritanopia makes someone unable to tell apart “blue and green, purple and red, and yellow and pink.”

The NHS notes that people usually adapt well. In children, signs include using the wrong colors when drawing and trouble with color-sorting tasks, and it can affect schoolwork. The NHS advises telling the school, which may be able to make adjustments for lessons and exams. It also says color vision deficiency may rule out certain careers for safety reasons, and gives pilot and train driver as examples.

A simulator can't tell you whether someone is color-blind: it only shows an estimate of which colors a given type may confuse. For a diagnosis, see an eye-care professional.

How common it is

The National Eye Institute says “about 1 in 12 men” have color vision deficiency. A 2012 review of population surveys by Jennifer Birch (Journal of the Optical Society of America A) gives about 8% of men and about 0.4% of women of European ancestry for the inherited red-green types, and 4% to 6.5% of men of Chinese and Japanese ethnicity.

What a simulator actually does

Most people have three types of cone cells, sensitive to long, medium and short wavelengths. In the full forms of color blindness one type is missing: protanopia (long, “red-blind”), deuteranopia (medium, “green-blind”) and tritanopia (short, “blue-blind”). Two colors that differ only in what the missing cone would have seen look the same to that person.

A simulator takes each color and replaces it with a color a typical viewer would confuse it with, chosen so that the two remaining cone types get the same signal. That is a prediction from a cone model. It tells you which colors collapse together, and nothing about how the remaining colors feel to someone who has seen this way all their life.

Three models are in common use:

• Brettel, Viénot and Mollon, 1997 (Journal of the Optical Society of America A). It projects colors onto two half-planes, and it works for all three types.
• Viénot, Brettel and Mollon, 1999 (Color Research and Application). A simplified version with a single plane, meant for the red-green types. libDaltonLens, which publishes its matrices, also gives one for tritanopia, but warns that “Viénot 1999 is not accurate for tritanopia. Use Brettel 1997 instead.” We include it below only for comparison.
• Machado, Oliveira and Fernandes, 2009 (IEEE Transactions on Visualization and Computer Graphics). A different physiological model with a severity value from 0 to 1, so it also covers the milder forms. At severity 1 it gives full protanopia and deuteranopia. For the blue-yellow type its authors model only the milder tritanomaly, as an approximation, and say they do not try to model tritanopia.

The DaltonLens review of open-source simulators compares these by eye and recommends Viénot 1999 for protanopia and deuteranopia and Brettel 1997 for tritanopia. Its update, based on feedback from color-blind readers in an r/ColorBlind thread, ranks Brettel and Viénot above Machado. The color blindness simulator on this site uses Viénot 1999 and Brettel 1997 the same way, and Machado 2009 for the milder red-green forms.

How far apart the three models are

We took 4,096 colors spread evenly over the sRGB range, ran each through all three models, and compared the outputs with ΔE.

For the red-green types, Brettel and Machado are close: a median ΔE of 2.1 for protanopia and 1.7 for deuteranopia. Viénot sits further from both, at medians of 4.9 to 6.3. Most of that gap is in blues and purples. For deuteranopia, the Viénot–Brettel median is 9.7 for colors where blue is the strongest channel and 5.7 for the rest. Viénot leaves pure blue #0000FF exactly as it is, while Brettel turns it into #0057FE. In the table of red and green pairs further down, Viénot and Brettel stay within 1 of each other, which is why either is fine for red-green checks.

For tritanopia, the three models disagree more: medians of 6.5 to 7.8 between any two, and the worst colors are ΔE 16 to 23 apart. Part of that gap is expected, since neither Machado nor Viénot is a tritanopia model. Bright green #11EE00 comes out as a light sky blue, #72D9FB, under Brettel and as a turquoise, #00E7CA, under Machado.

So the shade you see in a simulator depends on the model, by a visible amount.

Which colors merge: red-green models agree, blue-yellow ones less so

For design, the exact shade matters less than one question: will these two colors look the same? We tested that on the CSS named colors. They have 139 distinct values, and 9,275 pairs of them are clearly different to typical vision (ΔE 10 or more, so near-misses do not count). A pair counts as merging when its two simulated colors are less than ΔE 3 apart.

For protanopia and deuteranopia, every pair the models disagree on is a borderline case. Each time, the three models' ΔE values are less than 3 apart and sit on both sides of the merging line: blueviolet and royalblue, for example, are 3.2 under Viénot, 2.7 under Brettel and 2.6 under Machado. None of the 28 disputed pairs is a real split.

Tritanopia is different. Machado flags only 24 pairs, Viénot and Brettel 87 and 85, and in 40 of the 76 disputed pairs the models are 3 or more apart. Goldenrod and violet, for example, are ΔE 0.7 apart under Viénot and 0.9 under Brettel, but 12.0 under Machado. Most of the split comes from Machado, which does not model tritanopia. For this type, use Brettel 1997.

Some pairs that merge under all three models for deuteranopia: firebrick and green (ΔE 67 for typical vision, 1.4 to 1.8 simulated), coral and limegreen (1.2 in all three), dark green and dark red (2.4 to 2.7).

Familiar red and green pairs

The default red and green of matplotlib charts drop from ΔE 72 to under 5 for deuteranopia, and all three models agree within 0.1. That is past the merging line of 3 but short of the 6 at which colors look visibly different: treat the pair as too close to rely on. Tailwind's red-500 and green-500 keep about 8. They look different side by side, but stay under the 10 we count as clearly different, so a red or green status dot in these two colors still needs an icon or a label.

Pure red and CSS green stay about ΔE 15 apart for deuteranopia. Both turn olive yellow, but red becomes a lighter #A39000 and green a darker #776A18 (Machado). What merges is the hue; the difference in lightness survives.

All of these are full deuteranopia and protanopia, the worst case. In Machado's model the milder forms fall between typical vision and these values; we did not measure them here.

A common coding mistake: skipping one step

Screen colors are stored gamma-encoded. All three models are meant to run on linear light, so a correct simulator decodes sRGB first, applies the model and encodes again. Some code skips the decoding. The culori library did this in version 4.0.2, the version we checked in October 2026.

On the same 4,096 colors, Machado's deuteranopia matrix applied without decoding lands a median ΔE 4.8 from the correct result, with a maximum of 40.5. 38% of colors move by ΔE 6 or more. The typical shift is in the same range as the gap between models (medians of 1.7 to 6.3 for deuteranopia), but the worst cases are much larger: no two models are more than ΔE 15.1 apart for the red-green types. Red becomes a dark olive, #5E4700, where Machado computed correctly gives #A39000, a ΔE of 28. The three correct results for red are only ΔE 2.3 to 7.3 apart. The DaltonLens review makes the same point about missing sRGB decoding: it “makes a whole range of colors look way too dark.”

So if two simulators give you reds that are far apart, much more than ΔE 7, the model is an unlikely reason. Check whether one of them skips the decoding.

How to use a simulator well

1. Use it to find pairs that merge, and don't read too much into the exact shade. For red-green types the models agree on which pairs merge.
2. Check the exact colors you use, in a simulator that decodes sRGB first. The colorblind-safe palettes page shows tested sets for charts.
3. Treat blue-yellow results with more caution. Of the three models only Brettel 1997 is built for tritanopia, so prefer a tool that says it uses it.
4. When it matters, ask color-blind people. A model can tell you which colors are likely to merge, never how a design feels to use.
5. Don't rely on color alone. Labels, patterns and line styles don't depend on color at all. A difference in lightness helps too, and the contrast checker measures it as a WCAG ratio. For a red and green pair, read the simulated ΔE: below 3 it merges, below 6 it is too close to rely on, and below 10 it still needs an icon or label.

How we got these numbers

Colors: a 16 × 16 × 16 sRGB grid (every 17th value per channel), the CSS named colors from culori, and the pairs named above. Viénot 1999 and Brettel 1997 matrices come from libDaltonLens (public domain), Machado 2009 matrices from the authors' Table 1, all applied in linear RGB with the sRGB transfer function and clipped to the screen range. Full forms only (Machado at severity 1). Color difference is CIEDE2000 as implemented in culori, in CIELAB with a D65 white. Computed on 7 November 2026; the script also checks that its results match the simulator on this site for every grid color.

The formulas behind all our tools are on How We Calculate, and the Delta E calculator shows the color-difference scale.