Builds a harmony from one base colour: complementary, split-complementary, triadic, tetradic, square, analogous (count and angle adjustable) or monochromatic (lightness steps). Hue rotations run in perceptual OKLCH by default, with chroma reduced just enough to stay inside sRGB so every swatch keeps the intended hue; switch to HSL for classic colour-wheel maths. Output as CSS custom properties, a Markdown table or JSON, with swatches in the preview. Everything runs locally.