Reading
The guides
The converters answer what. These answer why, and when not to. Every example in them is rendered by the same functions the tools use, so the swatches you see are the values being described.
Color models
HSL versus RGB
Why the same color is easy to adjust in HSL and awkward in RGB, with the arithmetic behind both.
PrintWhen to use CMYK
What a CMYK value from any web tool can and cannot tell you about what comes off a press.
Modern CSSOKLCH explained
The perceptually uniform space CSS Color 4 added, and the practical problems it solves.
AccessibilityHow the WCAG contrast ratio is calculated
The relative luminance formula step by step, the thresholds it feeds, and what it misses.