huetension

A color palette toolkit for designers, frontend devs, and photographers. One binary, three modes — CLI, MCP server, and Web UI — sharing a single pure-Go core.

Table of Contents

• Features

• Three modes

  • CLI

  • MCP server

  • Web UI

• Install

  • go install

  • Pre-built archives

  • Docker

  • From source

• CLI quick reference

• MCP usage

• Web UI tour

• Configuration

• Library

• Development

• License

• Inspirations

• Acknowledgements

Related MCP server: MCP Color Converter

Features

🎨 Palette generation

• Extract from images — file path, http(s) URL, data: URI, or stdin. Methods: soft (default, designer-friendly), softk, kmeans, okkmeans, mediancut, octree, popularity, wu, dbscan, wkmeans, with mood presets for soft/softk.

• Random palettes — deterministic when seeded, optionally driven by a harmony rule.

• Harmonies around a base color — complementary, analogous, triadic, split, tetradic, square, double-complementary, compound, monochromatic, shades.

• Gradients — two endpoints or N positioned stops, interpolated in OkLab / OkLCH / Lab / RGB / HSL with optional easing.

• Interactive color wheel (web UI) — drag handles to mix custom palettes, pin the card while editing.

🔍 Analysis

• Contrast scoring — WCAG 2.1 and APCA side-by-side. Foreground "analyse" mode (--suggest) finds the nearest passing OkLCH lightness for a failing fg / bg pair.

• Color-vision-deficiency simulation — protanopia, deuteranopia, tritanopia, achromatopsia. Live preview in the SPA.

• Image color-distribution strips by color space and distance metric (analyze).

🛠️ Color utilities

• Convert between hex / RGB / HSL / HSV / HLS / Lab / LCH / OkLab / OkLCH / CSS named / 24-bit integer.

• Sort lists by luminance, lightness, OkLab L, hue, saturation, or frequency.

🎬 3D LUT

• .cube LUT — palette-driven, two grading methods: Smooth (RBF over OkLab) or Layered (K-NN over OkLCH).

• LUT Texture PNG — the cube laid out as a 2D image, handy for OBS-style real-time color grading and other tools that accept a texture LUT.

• In-browser preview on your own image — the backend pipes the .cube text into ffmpeg's lut3d filter, so ffmpeg must be on the server's PATH.

📚 Library

• Bundled curated catalogue ships in the binary.

• Save your own palettes to library.json from the CLI, the REST API, or the MCP library.save tool — they join the Saved category alongside the defaults.

📤 Export

• JSON, plain TXT, CSS custom properties, SCSS, LESS, Tailwind theme.extend.colors, GIMP .gpl, GIMP .ggr (gradient), SVG, PNG, JPEG, Adobe .ase, Adobe .aco.

Three modes

CLI

huetension extract photo.jpg -k 6
huetension harmony complementary "#3498db"
huetension gradient "#ff0066" "#00bcd4" --steps 9
huetension contrast "#222" "#fff" --algo wcag21

ANSI swatches by default, JSON / CSS / SCSS / Tailwind / GIMP .gpl / PNG / .ase / .aco on demand.

MCP server

huetension mcp --transport stdio

15 default-enabled tools (color.convert, harmony.generate, image.extract, library.list, …) speaking the same JSON contract as the CLI. Drop into Claude Desktop, an editor, or any MCP client:

{
  "mcpServers": {
    "huetension": {
      "command": "huetension",
      "args": ["mcp", "--transport", "stdio"]
    }
  }
}

Web UI

huetension web                       # http://127.0.0.1:8080

The embedded Vue SPA — color wheel, harmonies, image extraction, contrast / blindness checks, library, palette-driven 3D LUTs. Same REST envelope as huetension --format json.

Install

go install

go install github.com/leporel/huetension/cmd/huetension@latest

Requires Go ≥ 1.26. The SPA assets are committed under web/dist/ and embedded into the binary, so this produces a fully functional Web UI without bun.

Pre-built archives

Pick the right archive from the latest release:

Each archive ships the binary, LICENSE, README.md, and huetension.example.yaml. SHA-256 checksums are published next to the archives.

Docker

The release image is ghcr.io/leporel/huetension. Three common shapes:

Web UI + REST API —

docker run --rm -p 8080:8080 \
  ghcr.io/leporel/huetension:latest \
  web --address 0.0.0.0:8080

MCP over HTTP —

docker run --rm -p 7337:7337 \
  -v "$HOME/.config/huetension:/data:ro" \
  ghcr.io/leporel/huetension:latest \
  mcp --transport http --address 0.0.0.0:7337 \
      --auth-token "$TOKEN" --data-dir /data

Web + MCP on the same port (serve) —

docker run --rm -p 8080:8080 \
  -v "$HOME/.config/huetension:/data:ro" \
  ghcr.io/leporel/huetension:latest \
  serve --address 0.0.0.0:8080 \
        --auth-token "$TOKEN" --data-dir /data

The SPA is served at /, the REST API mounts under /api/v1, and the MCP HTTP transport mounts under /mcp. Add --mcp-transport http,sse to also expose SSE at /mcp/sse.

For MCP-over-stdio inside Docker (Claude Desktop and similar), see docs/mcp.md.

From source

git clone https://github.com/leporel/huetension
cd huetension/web && bun install --frozen-lockfile && bun run build
cd .. && go build ./cmd/huetension

Build order matters: the SPA assets are embedded, so bun run build must run before go build.

CLI quick reference

Full reference in docs/cli.md. Every command supports --help.

MCP usage

huetension mcp runs the MCP server over stdio (default; for Claude Desktop / editors), http (recommended for remote use), or sse (legacy). The same mcp: section of config.yaml controls which tools are exposed, and is also honoured by huetension serve.

Tool catalogue, transport details, image-sandbox flags, and container configs live in docs/mcp.md.

Web UI tour

Tools page (the default route):

• Color wheel — drag handles to mix custom palettes; pin the card to keep it visible while scrolling.

• Harmonies / Random / Gradient — generate around a base color or a seed.

• From image — drag-drop or paste an image; choose extract method + size.

• Contrast checker — WCAG / APCA side-by-side.

• Color blindness — preview palettes under each CVD type.

• Export — JSON / CSS / SCSS / LESS / Tailwind / GIMP .gpl / PNG / SVG / .ase / .aco. Includes a palette-driven 3D LUT tab with in-browser grading preview.

Library page (/library):

• Browse the bundled curated catalogue and any palettes you've saved.

Configuration

config.yaml lives in the data directory — ~/.config/huetension/ on Linux/macOS, %APPDATA%\huetension\ on Windows. Override with --data-dir or HUETENSION_DATA_DIR.

# Blacklist — start from the default-enabled set, remove specific tools.
mcp:
  disable:
    - image.extract
    - library.save

# Whitelist — expose ONLY these tools; everything else is off.
mcp:
  enable:
    - color.convert
    - harmony.generate
    - contrast.check

Precedence: flag > HUETENSION_* env > config.yaml > built-in default.

Full reference in docs/config.md; annotated example at huetension.example.yaml.

Library

The data directory also holds library.json. Bundled palettes ship with the binary; saved palettes (via huetension library add, POST /api/v1/library, or the MCP library.save tool) are appended under the Saved category.

{
  "version": 1,
  "palettes": [
    {
      "id": "brand-minimal",
      "name": "Brand minimal",
      "source": "user",
      "categories": ["Saved"],
      "colors": [
        { "hex": "#111827", "name": "Ink" },
        { "hex": "#F9FAFB", "name": "Paper" },
        { "hex": "#2563EB", "name": "Accent" }
      ]
    }
  ]
}

Development

Repo layout:

• cmd/huetension/ — CLI entrypoint, command tree.

• internal/color, internal/palette, internal/harmony, internal/gradient, internal/contrast, internal/blindness, internal/extract, internal/exporter, internal/imageio, internal/cliutil, internal/lut — pure-Go core.

• internal/web/ — REST API + embedded SPA.

• internal/mcp/ — MCP server + tool registry.

• internal/serve/ — combined SPA + REST + MCP listener.

• web/ — Vue 3 SPA source; built bundle committed to web/dist/.

• docs/ — long-form documentation.

Common tasks:

go test ./... -short          # fast tests (skips heavy extract sidecar)
go test ./...                 # full suite
cd web && bun run dev         # SPA dev server with HMR
huetension web --dev http://localhost:5173    # Go API in front of Vite

Release process: docs/RELEASING.md.

Extract testdata guidance: internal/extract/testdata/README.md.

License

MIT — see LICENSE.

Inspirations

Built after exploring these projects:

• qTipTip/Pylette — a Python library for extracting color palettes from supplied images.

• lazymac2x/color-palette-api — harmonies, gradients, WCAG contrast, CSS/Tailwind. REST API.

Acknowledgements

• modelcontextprotocol/go-sdk — MCP transport machinery.

• lucasb-eyer/go-colorful — color space conversions and distance metrics.

• anthonynsimon/bild — image decoding and processing helpers.

• charm.land/lipgloss — terminal styling for the CLI's ANSI swatches.

• spf13/cobra + spf13/viper — CLI / configuration plumbing.