JavaScript function to compute what color should be specified in CSS given desired transparency value and non-transparent color

equivalent_transparent_color.js

/**
 * Computes the color to be specified in CSS which will be visually equivalent to
 * <b>desiredColor</b> when the transparency is <b>alpha</b>.
 * Based on the method suggested in http://stackoverflow.com/questions/12228548/findinq-equivalent-color-with-opacity
 *
 * @options.backgroundColor -
 *    background color against which the computed background color will be overlayed
 * @options.desiredColor -
 *    desired color appearance in case alpha is 1
 * @options.alpha -
 *    the opacity for which the color is calculated
 */
function equivalentOpaqueColor(options) {
  var backgroundColor = options.backgroundColor;
  var desiredColor = options.desiredColor;
  var alpha = options.alpha;
  var equivalentColor = {
    toString: function() {
      var attributes = [this.r, this.g, this.b].join(', ');

      return 'rgb(' + attributes + ')';
    }
  };

  ['r', 'g', 'b'].forEach(function(attr) {
    var backgroundDifference = desiredColor[attr] - backgroundColor[attr];

    equivalentColor[attr] = (backgroundDifference + backgroundColor[attr] * alpha) / alpha;
  });
  return equivalentColor;
}

function rgb(r, g, b) {
  return {
    r: r,
    g: g,
    b: b
  };
}

var color = equivalentOpaqueColor({
  backgroundColor: rgb(255, 255, 255),
  desiredColor: rgb(180, 180, 180),
  alpha: 0.5
});

console.log('Equivalent color = ', color.toString());