Which colour group is least likely to appear harmonious?

Correct answer: Many unrelated bright colours

Harmony in colour generally depends on a recognisable relationship, such as neighbouring hues or a controlled family of colours. Blue with blue-green and green, red with red-orange and orange, and violet with blue-violet and blue each form related groups. Many unrelated bright colours lack a unifying relationship and may compete strongly for attention, so option D is least likely to appear harmonious.