PostgreSQL ENUM type

To create enum type

CREATE TYPE colors AS ENUM (‘grey’, ‘skyblue’, ‘black’);

To list enum values

SELECT ENUM_RANGE(null::colors); or  
SELECT UNNEST(ENUM_RANGE(null::colors)) AS colors;

To add new item on the enum list

ALTER TYPE colors ADD VALUE 'orange' AFTER 'skyblue';

To delete item from enum list

ALTER TYPE colors DROP attribute 'orange';

To drop enum type

DROP type colors

There is a more elegant way using domain to enforce value from David E. Wheeler post

CREATE DOMAIN eye_color AS TEXT
CONSTRAINT valid_eye_colors CHECK (
	VALUE IN ( 'blue', 'green', 'brown' )
);

CREATE TABLE faces (
	face_id SERIAL PRIMARY KEY,
	name TEXT NOT NULL DEFAULT '',
	eye_color eye_color NOT NULL
);