Check that a palette stays distinguishable under each deficiency

For normal vision and each deficiency at full severity, the palette's colours are converted to CIELAB and the smallest colour difference between any two of them is found. The difference is CIEDE2000 (Sharma, Wu & Dalal, 2005) unless metric asks for the older measure. The lower this minimum, the more likely two categories are to be confused. A palette counts as safe when the minimum across all four conditions is at least threshold.

Usage

palette_safety(colours = NULL, threshold = 5, metric = c("ciede2000", "cie76"))

Arguments

colours

The palette to test. Defaults to the depictr qualitative palette. Needs at least two colours: a pairwise distance over fewer than two has no value, rather than an infinitely safe one.

threshold

The smallest acceptable colour difference, on the scale of metric.

metric

The colour difference to measure: "ciede2000", the default, or "cie76", the Euclidean distance in CIELAB that earlier versions measured. "cie76" is kept so that earlier results can be reproduced, and a later release will remove it.

Value

A list with min_delta_e (the worst case across conditions), by_condition (a named numeric vector, one minimum colour difference for normal vision and for each deficiency), worst_condition and worst_pair (the closest colours and where they were closest), safe and threshold.

Details

Earlier versions measured the Euclidean distance in CIELAB, known as CIE76, which overstates the difference between saturated colours: the blues #005b96 and #0060a8 are 8.20 apart by CIE76 and 2.69 by CIEDE2000. ggplot2's default palette of five colours passed the check under CIE76, although under deuteranopia its sky blue and its pink come within 2.74 of each other in CIEDE2000.

The default threshold of 5 is calibrated against the reference colourblind-safe palette: the Okabe-Ito set's tightest pair (reddish purple against grey) sits at 6.43 under full deuteranopia, so the cut must lie below that to pass the recommended palette, while still flagging colours that become near-identical under a deficiency. The difference includes lightness, which survives colour-vision deficiency, so two colours that share a hue but differ in lightness are correctly treated as distinguishable. Full severity is the worst case; most colour-vision deficiency is milder.

This looks at a palette in the abstract. To audit a finished figure, which uses only as many colours as it has groups and has text and a background besides, see check_figure().

References

Okabe M, Ito K (2008). “Color Universal Design (CUD): How to make figures and presentations that are friendly to colorblind people.” https://jfly.uni-koeln.de/color/. Accessed 2026-06-14.

Machado GM, Oliveira MM, Fernandes LAF (2009). “A physiologically-based model for simulation of color vision deficiency.” IEEE Transactions on Visualization and Computer Graphics, 15(6), 1291–1298. doi:10.1109/TVCG.2009.113 .

Sharma G, Wu W, Dalal EN (2005). “The CIEDE2000 color-difference formula: Implementation notes, supplementary test data, and mathematical observations.” Color Research & Application, 30(1), 21–30. doi:10.1002/col.20070 .

Examples

palette_safety()
#> $min_delta_e
#> [1] 6.43
#> 
#> $by_condition
#> normal protan deutan tritan 
#>  21.25  13.44   6.43  11.13 
#> 
#> $worst_condition
#> [1] "deutan"
#> 
#> $worst_pair
#> [1] "#cc79a7" "#999999"
#> 
#> $safe
#> [1] TRUE
#> 
#> $threshold
#> [1] 5
#> 

# Two colours a hair apart are flagged.
palette_safety(c("#005b96", "#015c97"))
#> $min_delta_e
#> [1] 0.34
#> 
#> $by_condition
#> normal protan deutan tritan 
#>   0.35   0.36   0.34   0.34 
#> 
#> $worst_condition
#> [1] "deutan"
#> 
#> $worst_pair
#> [1] "#005b96" "#015c97"
#> 
#> $safe
#> [1] FALSE
#> 
#> $threshold
#> [1] 5
#> 

# ggplot2's default palette of five colours, which passed under CIE76.
hue5 <- c("#F8766D", "#A3A500", "#00BF7D", "#00B0F6", "#E76BF3")
palette_safety(hue5)$safe
#> [1] FALSE
palette_safety(hue5, metric = "cie76")$safe
#> [1] TRUE