What colours does the state flag of Hamburg have?

EDITORIAL LEARNING AID

Why is this answer correct?

The Hamburg state flag shows a white castle on a red background; its colours are white and red. Blue-white-red, green-white-red and black-yellow do not appear in the Hamburg flag.

Hamburg: white castle gate on red, colours white-red.

This translation and explanation are editorial learning aids. The official German wording and answer key govern the examination.