This tool evaluates color palettes for data visualization accessibility using three independent, validated methods: WCAG 2.1 contrast ratios, CIEDE2000 perceptual color difference (ΔE), and color vision deficiency (CVD) simulation. All calculations run entirely in your browser — no data leaves your machine.
The contrast ratio formula is defined in the WCAG 2.1 specification and uses relative luminance per IEC 61966-2-1 (sRGB). The linearization step — converting gamma-encoded 8-bit values to linear light — is the piece most implementations get wrong. This tool applies it correctly:
Verified values: Black on white = exactly 21:1 ✅ · #767676 on white = 4.54:1 ✅ (known borderline AA reference) · #595959 on white = 7.00:1 ✅ (AAA boundary)
WCAG 1.4.11 (Non-text Contrast) sets a 3:1 minimum for graphical objects including chart marks, icons, and UI components. This tool flags any palette color that fails 3:1 against both white and black — the worst-case condition.
ΔE measures perceptual distance between two colors in CIELAB color space, designed to be perceptually uniform — equal ΔE values correspond to roughly equal perceived differences. This tool uses CIEDE2000 (Sharma et al. 2005), the modern standard that improves on CIE76 with better accuracy in blue, near-neutral, and high-chroma regions. The implementation is validated against all 15 published reference pairs from Sharma 2005 Table 1 (all pass within 0.0005 tolerance).
Context thresholds are adapted from published data visualization accessibility guidance:
These thresholds are more demanding than generic WCAG thresholds because data visualization involves simultaneous color comparison across an entire chart, not just foreground/background reading.
Color vision deficiency simulations use the Machado, Oliveira & Fernandes (2009) model, which is the most accurate published approach for simulating the full spectrum of dichromacy and anomalous trichromacy. The matrices are applied in linearized RGB space (not gamma-encoded), which is the correct workflow.
Verified behaviors:
Research on pre-attentive visual processing and categorical color discrimination consistently recommends no more than 8 colors for a qualitative data visualization palette intended to be decoded simultaneously. ColorBrewer caps its qualitative sets at 8–12; Edward Tufte and Stephen Few recommend 5–7 for most chart contexts. This tool flags palettes above 8 colors and warns above 12.
Uses CIEDE2000 ΔE (modern standard, validated against Sharma 2005). CVD simulation represents average dichromatic and anomalous trichromatic vision — individual variation exists. All results are advisory; final accessibility decisions should involve testing with real users and screen readers.