function luminosity(color){
  utils.assertColor(color);
  color = color.rgba;
  function processChannel(channel) {
    channel = channel / 255;
    return (0.03928 > channel)
      ? channel / 12.92
      : Math.pow(((channel + 0.055) / 1.055), 2.4);
  }
  return new nodes.Unit(
    0.2126 * processChannel(color.r)
    + 0.7152 * processChannel(color.g)
    + 0.0722 * processChannel(color.b)
  );
}
Returns the relative luminance of the given `color`,
see http://www.w3.org/TR/WCAG20/#relativeluminancedef

Examples:

    luminosity(white)
    // => 1

    luminosity(#000)
    // => 0

    luminosity(red)
    // => 0.2126

@param {RGBA|HSLA} color
@return {Unit}
@api public