Color blindness check
WCAG 1.4.1 is not about contrast. Two colors can clear 4.5:1 and still be indistinguishable to one man in twelve. This finds those pairs and separates them.
Normal vision
What you see
Deuteranopia
Green looks like red
collapses · 0.07
Protanopia
Red looks like green, and darker than it should
collapses · 0.10
Tritanopia
Blue and yellow collapse together
stays apart · 0.34
These two collapse into one color
Anywhere these carry meaning on their own, a red dot against a green one, a positive line against a negative one, that meaning is lost. WCAG 1.4.1 is the criterion for this, and it is not about contrast: both colors can clear 4.5:1 against the page and still fail here.
Dichromatic vision keeps lightness discrimination, so the fix is to separate them in lightness. This is the nearest color that stays apart under all three deficiencies at once, with your hue held exactly.
Simulated with the Machado, Oliveira and Fernandes (2009) model at full severity, applied in linear RGB. Distances are OKLab delta E, the same measure this site uses for how gentle a fix is.
Why this rule exists
The other tools
Contrast checker
Two colors, the ratio, and the nearest color that passes.
Palette matrix
Every pair in your palette, checked at once.
Accessible ramp
One brand color, a full scale, every step measured.
Every passing color
Not one fix. The whole range of your color that clears the rule.
Focus and UI check
Fifteen interface parts, and which of them the 3:1 rule exempts.