no output when usgin postcss-color-function

[cmnstmntmn]

--color-darker: color(red shade(50%);

this is the css config

postcss([  
    require('postcss-import')(),
    require('postcss-mixins')(),
    require('postcss-nested')(),
    require('postcss-style-guide')({
      project: 'My Project',
      dest: 'styleguide/app.html',
      showCode: true
    }),    
    require('postcss-cssnext')({
      features: {
        customProperties: {},
        applyRule: {},
        calc: {},
        customMedia: {},
        mediaQueriesRange: {},
        customSelectors: {},
        attributeCaseInsensitive: {},
        colorRebeccapurple: {},
        colorHwb: {},
        colorHsl: {},
        colorRgb: {},
        colorGray: {},
        colorHexAlpha: {},
        colorFunction: {},
        fontVariant: {},
        filter: {},
        initial: {},
        rem: {},
        pseudoElements: {},
        pseudoClassMatches: {},
        pseudoClassNot: {},
        pseudoClassAnyLink: {},
        colorRgba: {},
        overflowWrap: {},
        autoprefixer: false      
      }
    }),      
    require('postcss-typescale'),
    require('postcss-button')    
  ])
  .process(fs.readFileSync('resources/games/css/app.css', 'utf8'), { from: "resources/games/css/app.css" })
  .then(result => purify(content, result.css, options))
  .catch(error => {
    console.error(error);
  });