Create a sum type for colors in sql

postgresql_color_adt.sql

-- data NamedColor = Red | Green | Blue
-- data CustomColor = CustomColor
--  { r :: Int
--  , g :: Int
--  , b :: Int
--  }
-- data Color = NamedColor | CustomColor

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

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 $$
  declare
    custom_int integer;
    named_int integer;
  begin
  select case when c.named is null then 0 else 1 end into named_int;
  select case when c.custom is null then 0 else 1 end into custom_int;
  return custom_int + named_int = 1;
  end;
$$ 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('green', '(0,7,255)')); -- failure

select * from color_tester;

select json_strip_nulls(row_to_json(color_tester)) from color_tester;
-- where color = row('red',null)::impl_color;