PostgreSQL function checking whether a parameter is in a valid set of values

color_check.sql

--
-- Function that needs to explicitly check whether the 'color' value is valid
--
CREATE OR REPLACE FUNCTION get_gluon(color text) RETURNS TEXT AS $$
#print_strict_params on
DECLARE
  duck TEXT;
BEGIN
  IF color not in ('red', 'green', 'blue') THEN
     RAISE EXCEPTION 'unknown color %', color;
  END IF;
  -- ....
  return duck;
END;
$$ LANGUAGE plpgsql
IMMUTABLE
RETURNS NULL ON NULL INPUT
PARALLEL SAFE;