extractRgbSubstring = function extractRgbSubstring(string) {
  var candidate = string.match(matchRgb);
  if (
    candidate &&
    candidate.length > 1 &&
    candidate[1].length > 0 &&
    typeof candidate[1] === 'string'
  ) {
    return candidate[1];
  }
  return '0,0,255';
}
Extracts the comma-separated numbers from a rgb(a?) string.
@param string
@returns {string}