from the left table that do not exist in the right table.....

CREATE TABLE palette_a (
    id INT PRIMARY KEY,
    color VARCHAR2 (100) NOT NULL
);
CREATE TABLE palette_b (
    id INT PRIMARY KEY,
    color VARCHAR2 (100) NOT NULL
);
INSERT INTO palette_a (id, color)
VALUES (1, 'Red');

INSERT INTO palette_a (id, color)
VALUES (2, 'Green');

INSERT INTO palette_a (id, color)
VALUES (3, 'Blue');

-- insert data for the palette_b
INSERT INTO palette_b (id, color)
VALUES (1, 'Green');

INSERT INTO palette_b (id, color)
VALUES (2, 'Red');

INSERT INTO palette_b (id, color)
VALUES (3, 'Cyan');

select a.id a_id, a.color colour, b.id b_id, b.color colour
from palette_a a left join palette_b b on a.color = b.color
where b.id is null;