Create a sum type for colors in sql

postgresql_color_adt.sql

create type custom_color as (r smallint, g smallint, b smallint);

create type named_color as enum ('red', 'green', 'blue');

create type color as (named named_color, custom custom_color);

create or replace function check_color(c color) returns boolean as $$
  begin
  return
  	(c.named IS NOT NULL)::int +
  	(c.custom IS NOT NULL)::int = 1;
  end;
$$ language plpgsql;


create table color_tester (
  color color not null default '(red,)' CHECK(check_color(color))
);

insert into color_tester (color) values(row(null, '(0,7,255)')); -- success
insert into color_tester (color) values(row(null, '(250,7,255)')); -- success
insert into color_tester (color) values(row('green', null)); -- success
insert into color_tester (color) values(row('red', null)); -- success
-- insert into color_tester (color) values(row(null, null)); -- failure
-- insert into color_tester (color) values(row('green', '(0,7,255)')); -- failure


select * from color_tester;

select json_strip_nulls(row_to_json(color_tester)) from color_tester;