Who this affects
Around 8% of men and 0.5% of women of Northern European descent have some form of colour vision deficiency. The overwhelming majority is red-green: deuteranomaly alone accounts for about 6% of men, which makes it more common than left-handedness in that group.
Blue-yellow deficiency and complete colour blindness exist but are genuinely rare. If you only check one simulation, check deuteranomaly.
What usually breaks
Red-on-green status indicators are the classic failure — the single most common way to signal success and error, and the single most invisible to the largest affected group. Charts relying on a rainbow palette collapse into a few indistinguishable bands. Red 'required field' text stops looking urgent.
The fix is never to abandon colour, only to stop relying on it alone: add an icon, a label, a shape, a pattern, or a difference in lightness. Lightness contrast survives every simulation on this page.
How the simulation works
Each vision type has a matrix that redistributes the red, green and blue channels to approximate what those cones actually deliver. These are the Machado, Oliveira and Fernandes (2009) transforms at full severity — the same model behind most browser and design-tool accessibility features.
It is an approximation, not a window into someone's experience. Colour perception is shaped by adaptation and context that no matrix captures. Treat it as a design check, not a simulation of a person.
Frequently asked questions
My chart still looks fine in the simulation — am I done?
It is good evidence, not proof. Also check the contrast between adjacent colours with a contrast checker, and make sure nothing depends on colour alone. A legend that relies on matching swatches to lines is a problem even when the colours survive.
Are my images uploaded?
No. The simulation runs on the pixels in your browser, so an unreleased design is not uploaded to be checked.
Something wrong with this tool, or an idea for it? Tell us