PostgreSQL function using user-defined enum type in case statement

get_gluon.sql

CREATE OR REPLACE FUNCTION get_gluon(color color) RETURNS TEXT AS $$
#print_strict_params on
DECLARE
  duck TEXT;
BEGIN
  CASE color
    WHEN 'RED'::color THEN
      duck := 'Huey';
    WHEN 'GREEN'::color THEN
      duck := 'Dewey';
    WHEN 'BLUE'::color THEN
      duck := 'Louis';
    ELSE
      RAISE EXCEPTION 'unknown color %s', color;
  END CASE;
  return duck;
END;
$$ LANGUAGE plpgsql
IMMUTABLE
RETURNS NULL ON NULL INPUT
PARALLEL SAFE;