CREATE TYPE t_color AS ENUM
   ('WHITE',
    'BLACK',
    'GREEN',
    'BLUE');
ALTER TYPE t_color OWNER TO appuser;

CREATE OR REPLACE FUNCTION get_f1(IN f1 t_color)
RETURNS XML AS
$BODY$
SELECT XMLELEMENT(name color, XMLATTRIBUTES($ 1  AS selected));
$BODY$
LANGUAGE 'sql' IMMUTABLE STRICT;
ALTER FUNCTION get_f1(IN f1 t_color) OWNER TO appuser;

CREATE OR REPLACE FUNCTION get_f2(IN f2 t_color) RETURNS XML AS
$BODY$
BEGIN
RETURN XMLELEMENT(name color, XMLATTRIBUTES(CAST(f2 AS t_color) AS selected));
END;
$BODY$
LANGUAGE 'plpgsql' IMMUTABLE STRICT;
ALTER FUNCTION get_f2(IN f2 t_color) OWNER TO appuser;

SELECT XMLELEMENT(name color, XMLATTRIBUTES(CAST('WHITE' AS t_color) AS selected)),
       XMLELEMENT(name color, XMLATTRIBUTES(CAST(NULL AS t_color) AS selected)),
       get_f1(CAST('WHITE' AS t_color)),
       get_f1(CAST(NULL AS t_color)),
       get_f2(CAST('WHITE' AS t_color)),
       get_f2(CAST(NULL AS t_color))

--Output
--"<color selected="WHITE"/>";"<color/>";"<color selected="WHITE"/>";"";"<color selected="WHITE"/>";""