About ToolQuilt: how our color tools work

The color math

Everything is calculated on the server from first principles, not by a third party library, so the working can be inspected and corrected.

Contrast

The WCAG 2.2 ratio is (L1 + 0.05) / (L2 + 0.05), where L is relative luminance computed from linearized sRGB channels with the standard 0.2126 / 0.7152 / 0.0722 weights. Black on white gives exactly 21:1, which is the sanity check every implementation should pass.

The APCA score uses the published 0.1.9 constants: a 2.4 exponent transfer curve, separate coefficients for the two polarities, a soft black clamp at 0.022, and the low contrast clip at 0.1. Black text on white returns roughly Lc 106; white text on black returns roughly Lc −108. APCA is a candidate for WCAG 3 and is not yet a legal standard, so conform to WCAG 2.2 and use APCA as the second opinion that catches pairs which pass the ratio but still read badly.

Color spaces

Conversions go through CIE XYZ under a D65 white point with the 2 degree standard observer. CIELAB and LCH follow the CIE definitions, including the linear segment below the 0.008856 threshold. OKLab and OKLCH use Björn Ottosson's published matrices. Round tripping any color through LAB or OKLCH and back returns the original hex.

CMYK is the simple arithmetic conversion with no color profile attached. It will get you into the right neighborhood and no further. Real print work needs an ICC profile for the specific press, paper and ink set.

Palettes and scales

Harmony rules set the hue relationships. Lightness and chroma are then spaced again in OKLCH, because HSL lightness does not match perceived lightness: yellow at 50% in HSL looks far brighter than blue at 50%. The 50 to 950 tonal scale holds hue, walks a fixed lightness ladder, and shapes chroma with a curve that peaks in the middle, which is what stops the lightest and darkest steps looking artificial.

Color vision simulation

The protanopia, deuteranopia and tritanopia previews use the Machado, Oliveira and Fernandes (2009) matrices at full severity, applied in linear RGB. They are a reasonable model, not a medical instrument. Treat them as a prompt to add a second signal, such as a label, an icon or a pattern, wherever two swatches converge.

Privacy, in short

The full detail is in the privacy policy and the cookie policy. The summary:

• No account system, so there is nothing to sign up for and no profile to leak.
• Colors you enter travel in the URL query string so results are shareable. They are processed to build the page and are not written to any database.
• A single browser localStorage entry records whether you chose the light or dark theme. It never leaves your device.
• No analytics script, no session recording, no fingerprinting.
• No advertising network, and so no advertising cookies.
• No third party fonts, embeds or social widgets. Every asset is served from this domain.
• Standard web server request logs may exist at the hosting layer, as they do for every website.

Standards this follows

• WCAG 2.2, Success Criteria 1.4.3, 1.4.6 and 1.4.11.
• APCA 0.1.9, as drafted for WCAG 3.
• CSS Color Module Level 4 for parsing and output syntax.
• CIE 15:2004 for XYZ, LAB and LCH.

Corrections

If a number here disagrees with a reference implementation, the number here is the one to doubt. Color math has a lot of places to drop a coefficient, and every tool on this site shows its inputs so a disagreement can be traced.