@saab,
Wilhelm Hasselmann (
Socialist Workers Party, which became the
General German Workers' Association, then Social Democratic Party) was the first editor for the Social-Democratic "Die Rote Fahne" (
The red flag) in 1876/77. (He later emigrated and ,in 1888, became an American citizen.)
The association of a red flag with workers goes back to 1848 revolution.
And thus, we've got the red colour in our national flag (which is the one from 1848).