Monochrome mystery / Monochromes Geheimnis

A couple of hundred years ago, when Goethe was at the peak of his writing, there was little distinction between art and science. In his view his most important work was not his poetic Faust, but rather his ‘scientific’ Theory of Colours.

In it, as simplified by Wikipedia, he “contentiously characterized colour as arising from the dynamic interplay of light and darkness through the mediation of a turbid medium”. And that’s the easy version!

I cross Goethestraße daily, its impressive buildings sadly marred by speeding cars - but I have noticed this intriguing monochrome flag. I’m pretty sure it’s not a nation or a region, so my best guess is some kind of historical Prussian influence - any theories?