code: function getColorLightenFunction(less) {
  
    function rgba() {
      var rgbaFunc = less.functions.functionRegistry.get("rgba");
      return rgbaFunc.apply(null, arguments);
    }
  
    return function (color, amount) {
      var baseRGB = [255, 255, 255];
      var value = amount.value / 100;
  
      if (value < 0) {
        baseRGB = [0, 0, 0];
        value = -1 * value;
      }
  
      return rgba(
        Math.round((baseRGB[0] - color.rgb[0]) * value) + color.rgb[0],
        Math.round((baseRGB[1] - color.rgb[1]) * value) + color.rgb[1],
        Math.round((baseRGB[2] - color.rgb[2]) * value) + color.rgb[2],
        color.alpha
      );
    };
  }
docs: USAGE: color-lighten(@neutral, 50);