JAVASCRIPT

convert hex to rgba with opacity

//convert hex to rgba with opacity
function addAlpha(color, opacity) {
    // coerce values so ti is between 0 and 1.
    var _opacity = Math.round(Math.min(Math.max(opacity || 1, 0), 1) * 255);
    return color + _opacity.toString(16).toUpperCase();
}
addAlpha('FF0000', 1); // returns 'FF0000FF'
addAlpha('FF0000', 0.5); // returns 'FF000080'

white with opacity rgba to hex

function hexify(color) {
  var values = color
    .replace(/rgba?(/, '')
    .replace(/)/, '')
    .replace(/[s+]/g, '')
    .split(',');
  var a = parseFloat(values[3] || 1),
      r = Math.floor(a * parseInt(values[0]) + (1 - a) * 255),
      g = Math.floor(a * parseInt(values[1]) + (1 - a) * 255),
      b = Math.floor(a * parseInt(values[2]) + (1 - a) * 255);
  return "#" +
    ("0" + r.toString(16)).slice(-2) +
    ("0" + g.toString(16)).slice(-2) +
    ("0" + b.toString(16)).slice(-2);
}

var myHex = hexify('rgba(255,255,255,0.6)'); // "#f5faf3"

console.log(myHex);