<===> input.scss
$quoted: "green";
$unquoted: green;

.test {
  string: type-of($quoted);
  color: type-of($unquoted);
  string: type-of("green");
  color: type-of(green);
}

<===> output.css
.test {
  string: string;
  color: color;
  string: string;
  color: color;
}