Two separate questions about category colors
When several bars or lines are told apart by color there are two things to check: whether each color shows up against the chart background, and whether the colors are distinguishable from one another. These need different measures, yet they are often collapsed into one. This tool reports a background contrast ratio per color and a lightness gap per pair, side by side.
How it is calculated
Contrast uses the relative luminance formula from W3C WCAG. The non-text 3:1 check is applied only between a color and the background, never as a test for separating two category colors. Pair separation is measured as the difference in CIELAB lightness L*, and because no official warning threshold exists the tool judges against the value you enter. The eight colors proposed by Okabe and Ito load with one button as a reference palette. Written as of October 2026.
Limits
Color-vision-deficiency simulation and safety scores or grades are not supported. Pairs with a small lightness gap rely on hue alone, so add outlines, patterns or direct labels. The tool does not recommend palettes for you.
Frequently asked questions
No. The non-text 3:1 requirement compares a graphic element with its background or an adjacent color. Even the Okabe-Ito palette, chosen for color-vision deficiency, has 20 of its 28 pairs under 3:1 with each other. Pair separation comes from hue and lightness difference.
The eight colors proposed by Okabe and Ito were picked so they stay distinguishable under common color-vision deficiencies, which makes them a good starting point. On a light background the orange, sky blue and yellow end up with low background contrast, so check the ratios in the table.
There is no official value, so the tool takes the threshold from you and judges only against what you entered. It does not produce a score or a grade.