Cologne lies by the Rhine river, so we need bridges. They are all beautiful and green — chromium oxide green, more precisely, which, in fact, is known as Cologne green. Lord Mayor Konrad Adenauer wanted a patina-like green color and in 1929 Bayer AG mixed the color, which is particularly light and weather resistant.

http://manchesterhistory.net/architecture/1940/severin.html
https://en.wikipedia.org/wiki/Chromium(III)_oxide
https://www.guiding-architects.net/en/destinations/cologne/