Heligoland – Flag in Lexicon and Shop https://www.flaggenlexikon.de/fdthelgo_me.htm
Heligoland Island belonged to Denmark from 1773 to 1814, to Great Britain from 1814, to the German Empire from 1890 and since 1952 to the Federal Republic of Germany. Its flag – which can also be ordered on this page – shows three horizontal stripes in green, red and white.
The colours have their roots in the crest of the island