A palette stays distinguishable under color blindness by lightness contrast, not by hunting for safe hues

What it is

The dependable way to keep a categorical palette readable under colour vision deficiency is not hunting for "hues the colour blind can still tell apart" — it is ensuring every pair of categories keeps a sufficient lightness difference. Hue axes can be compressed by deficiency; the lightness axis survives in nearly every colour-vision type. Two colours pulled apart in depth stay in two classes even for readers whose red-green axis has failed; two colours differing only in hue at equal depth can merge into one at any moment.

Why it happens

Colour deficiency compresses particular directions of colour space (most commonly the red-green axis), while the lightness signal is built from a weighted combination of all cone responses and processed through the luminance channel — a path distinct from hue comparison and essentially untouched by red-green variants. The guarantee of separability therefore migrates from hue to lightness: sort the candidate palette by lightness, require every adjacent step to exceed the discrimination threshold, and let hue add the finishing touch. This also explains why the cheapest engineering fix for a red-green pair is a lightness step (deep red against pale green): the semantics stay, only the depths separate.

Where it stops holding

A lightness step is the insurance policy for separability, not a decoration: with many categories, spreading them purely across lightness degrades the palette into an ordered greyscale ramp, slows legend matching, and risks collisions with backgrounds and gridlines at the dark and pale ends. Lightness safety is also not sequential-encoding correctness — ordering categorical data into a brightness ladder invites readers to read magnitude into it, which is when shape or pattern must step in rather than lightness being squeezed harder.

Applying it

• Build categorical palettes lightness-first: sort candidates by L*, and eliminate or retune any adjacent pair below the discrimination threshold.
• When red and green must coexist, force one dark and one pale (deep red/pale green or pale red/deep green), and verify simulation does not invert the depth relation.
• Re-validate lightness steps on dark backgrounds: the same palette separates differently on dark versus light surfaces, so a light-background check does not carry over.
• Verification: run the palette through simulation and greyscale conversion and confirm every pair still separates; then time real readers on legend matching and widen the lightness step for any pair that slows them down.

Related

• Same group: U4.04.1 Red-green combinations fail for a substantial share of viewers · U4.04.3 Must remain readable in greyscale
• Nearby: U4.01.3 Shape or pattern redundancy is required alongside colour · U4.02.3 Lightness is the main carrier of perceived order
• Search terms: lightness separation · CVD safe palette · categorical colour