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CVD accessibility audit — hdatools palettes

Source: vignettes/articles/cvd-audit.Rmd
cvd-audit.Rmd

This article documents a colorblind-vision (CVD) audit of the four hdatools brand palettes conducted for the 0.4.0 release. It covers protanopia, deuteranopia, and tritanopia at full severity. No palette color order was changed as a result of this audit (see plans/DECISIONS.md, Q7, 2026-07-18). The findings here inform chart authors on which color pairs to avoid in categorical plots and when supplemental encodings are needed.

Method

Pairwise perceptual distance (delta-E, CIE76) was computed in Lab color space via colorspace:

  1. Simulate full-severity CVD with colorspace::protan() / deutan() / tritan()
  2. Convert simulated hex values to Lab with colorspace::hex2RGB() → as(., "LAB")
  3. Compute Euclidean distance (delta-E) for every color pair

Delta-E below 10 is generally considered indistinguishable; values of 10–20 are borderline; values above 20 are comfortably distinct. The audit focuses on the first four palette slots — the range almost always used in a categorical plot — plus two pairs flagged during the 0.4.0 design review.

HDA

Palette: Blue #445ca9, Green #8baeaa, Yellow #e9ab3f, Coral #e76f52, Lavender #a97a92, Sea Green #8abc8e

HDA palette — pairwise delta-E by CVD type
color1 color2 Normal Protan Deutan Tritan Flag
Blue Green 57.7 50.9 51.9 29.5
Blue Yellow 111.0 109.7 113.8 73.8
Blue Coral 89.4 73.5 91.8 95.6
Green Yellow 68.8 63.0 65.0 57.3
Green Coral 70.0 33.1 45.2 85.1
Yellow Coral 41.7 37.3 22.7 30.8

First-4 audit — no failures. The minimum pairwise delta-E across all CVD types is 22.7 (Yellow vs Coral under deuteranopia). All first-4 pairs are comfortably distinct.

HDA: Green vs Sea Green (flagged pair)

HDA — Green vs Sea Green palette — pairwise delta-E by CVD type
color1 color2 Normal Protan Deutan Tritan Flag
Green Sea Green 24 22.3 19.6 6 ⚠️

Tritanopia failure confirmed. Green (#8baeaa) and Sea Green (#8abc8e) collapse to delta-E = 5.97 under full tritanopia — effectively indistinguishable. Under protanopia and deuteranopia the pair is acceptable (delta-E ≈ 20–22).

Tritanopia affects approximately 0.1 % of the population. Green and Sea Green are positions 2 and 6, so they appear together only in a 5–6-category plot. Recommendations:

  • Avoid pairing Green and Sea Green in a plot that targets tritanopic accessibility.
  • In 2–4-category plots the pair will not both appear, so no action is needed.
  • If a chart must use both and CVD robustness is critical, replace one with a manual override (e.g. swap Sea Green for Coral or Lavender).

HFV

Palette: Shadow #334a66, Sky #66cccc, Lilac #a29dd4, Grass #50aaa7, Berry #c0327e, Desert #ec7c53, Leaf #6fb547, Cerulean #7fc7e0

HFV palette — pairwise delta-E by CVD type
color1 color2 Normal Protan Deutan Tritan Flag
Shadow Sky 55.0 49.1 45.3 52.4
Shadow Lilac 39.6 36.9 37.7 38.3
Shadow Grass 45.0 37.9 34.3 40.0
Sky Lilac 47.7 26.5 17.2 39.7
Sky Grass 12.4 12.6 12.1 12.7
Lilac Grass 46.0 25.4 18.4 34.5

Sky vs Grass is the structurally risky pair. Delta-E is approximately 12 under all three CVD types (12.1–12.7) — within the borderline range and the lowest minimum of any first-4 audit. Both colors are teal-family; they are distinguishable in normal vision by their lightness contrast but that contrast diminishes under red-green CVD.

No pair falls below 10 in the first-4 audit, so no automatic failure. Recommendations:

  • Avoid using Sky and Grass as the only two series colors in a CVD-sensitive chart. If both are needed, add a shape, pattern, or direct-label encoding alongside color.
  • 3-category or 4-category plots should order series so Sky and Grass are not adjacent in a legend (where label proximity compensates for color proximity).

PHA

Palette: Green #5bab8e, Light Blue #a6cccc, Orange #f39152, Red #be451c, Purple #a5add0, Dark Blue #2b6b9c

PHA palette — pairwise delta-E by CVD type
color1 color2 Normal Protan Deutan Tritan Flag
Green Light Blue 26.9 19.8 18.5 21.3
Green Orange 75.6 32.0 47.6 86.3
Green Red 90.1 39.5 48.2 105.9
Light Blue Orange 69.5 48.7 57.0 72.8
Light Blue Red 85.7 59.3 63.2 96.3
Orange Red 27.8 25.6 22.5 28.9

Green vs Light Blue is the tightest pair (delta-E ≈ 18.5–21.3 across CVD types). Both are cool desaturated colors that lose further differentiation under red-green CVD. No pair falls below 10.

PHA: Orange vs Red (flagged pair)

PHA — Orange vs Red palette — pairwise delta-E by CVD type
color1 color2 Normal Protan Deutan Tritan Flag
Orange Red 27.8 25.6 22.5 28.9

Not confirmed as a failure. The flagged Orange/Red pair maintains delta-E ≥ 22 under all CVD types; the lightness difference (Orange is substantially lighter) provides the primary perceptual cue and it is preserved under CVD simulation.

Recommendation: no action needed for the Orange/Red pair. For Green vs Light Blue in 4-category plots, consider adding a secondary encoding if tritanopic accessibility is a concern.

VHA

Palette: Dark Turq #0C4D4F, Light Green #A0D18E, Yellow #ECC51E, Light Turq #19787B, Grey #2E3030, Light Blue #E3F3F5

VHA palette — pairwise delta-E by CVD type
color1 color2 Normal Protan Deutan Tritan Flag
Dark Turq Light Green 61.7 61.2 61.0 49.0
Dark Turq Yellow 100.6 95.8 100.1 72.2
Dark Turq Light Turq 17.9 17.1 16.5 18.3
Light Green Yellow 57.0 47.0 49.8 46.2
Light Green Light Turq 50.3 49.8 51.2 34.8
Yellow Light Turq 97.0 90.1 95.6 68.5

Dark Turq vs Light Turq is the tightest pair (delta-E ≈ 16.5–18.3 across CVD types). Both are deep teal tones; the primary cue under CVD is the lightness difference (Dark Turq is considerably darker). No pair falls below 10 in the first-4 audit.

Recommendations:

  • In 2-category plots the pair is acceptable; the lightness contrast is sufficient.
  • In 4-category plots, prefer not to place Dark Turq and Light Turq as the only two categories without supplemental encoding.

Diverging ramp caveat

As documented in NEWS.md, 7-class diverging maps built from any of the six continuous ramps lose sign distinction in the innermost class pair under protanopia, because both arms converge toward the shared cream center. This is structural to any cream-centered diverging ramp; the mitigation is to always pair a palette = "diverging" map with a legend or direct class labels.

Regression tests

tests/testthat/test-cvd.R contains colorspace::simulate_cvd()-based assertions that the first four colors of each brand palette maintain the minimum pairwise delta-E thresholds observed above. These tests guard against future palette edits that silently reduce CVD distinguishability; they do not validate the absolute CVD safety of the palettes, and they do not test the HDA Green/Sea Green tritanopia failure (which is a documented limitation, not a threshold a future edit could improve accidentally).