My previous theory that it was all dreamed up by the advertising men taking advantage of colour supplements in the 1960s was completely wrong. It’s significantly older – already everywhere in the 1950s:
A refreshing beverage for those with a taste for tartness is "weisse mit schuss," the summertime favorite in Berlin. The "weisse" is a markedly sharp beer brewed from malted wheat rather than the customary barley. The "schuss" is a natural heavy raspberry syrup.
– New York Times, May 12 1953
This is the season of the year when outwardly tranquil West Berliners, by the tens of thousands, sit contentedly in innumerable beer gardens and crowded sidewalk cafés and Drink “Weiss mit Schuss”.
To the casual eye, the West Berliner has not a care in the world as he sits with his nose in a great glass bird bath, approximately five times the size of a champagne glass, and quaffs Berlni’s special drink, “Weiss mit Schuss”. This is a special kind of “white” beer spiked with a slug of raspberry juice. It sounds horrendous. But it is surprisingly palatable on a hot day and Berlin, like much of the rest of Europe, has been sweltering through one of the hottest summers on record.
– Reading Eagle, July 20 1959 (typos in original)
Before the war, it’s mentioned too:
Mit dem Schuß Gemütlichkeit, der zum richtigen Berliner gehört, wie eben der Schuß Himbeer zur richtigen Weiße. (He had the dash of friendliness that belongs to a proper Berliner, just as a dash of raspberry belongs in a proper white beer.)
Paul Westheim, Helden und Abenteurer: Welt und Leben der Künstler, H. Reckendorf 1931, p. 165
In einer benachbarten Kneipe wurden unzählige Weiße mit Himbeer getrunken. (In a neighbouring bar countless white beers with raspberry were drunk.)
George Grosz, Ein kleines Ja und ein großes Nein, Rowohlt 1955(Grosz emigrated in 1933 so this passage must relate to something before then.)
Man konsumiert Gose wie die Berliner Weiße: mit Himbeer („Schuß") oder indem man Kümmel mittenmang hinter die Binde schüttet. (One consumes Gose like Berliner Weisse: with raspberry (Schuss), or by downing shots of caraway schnapps alongside.)
Hans Reimann, Das Buch von Leipzig, Leipzig 1929
Fr Finton Stack also pointed out in a comment that the characters in Döblin’s classic Berlin Alexanderplatz (1929) also drink Weisse both red and green.
So it goes back at least to the 1920s, and seems to have been fairly well established by then. How much further? Possibly as long as the syrup has been industrially manufactured. In the first decade of the 20th century there were already ready-made “essences” that you could buy to flavour cold wine punches such as the woodruff-flavoured Maibowle.
We can go back further still. Just found this tempting snippet:
Ich nehme dann belegte Stullen in der großen Strohtasche dort mit. Die werde an Ort und Stelle im Gasthaus verzehrt, und dazu geht eine “jroße Weiße mit” rundum. Mit was denn? Mit Himbeer. Mit Himbeer? Ja. Ein unglaubliches Gemisch, aber sehr beliebt. Ein großes Schnapsglas voll Himbeersaft wird in das Weißbier gegossen. Wenn man ein bis zwei Stunden marschirt ist und Durst hat, schmeckt alles.
( I took sandwiches with me. They were eaten on the spot in the pub. As accompaniment a “large white with” goes around. With? With what? Raspberry. Raspberry? Yes. An unbelievable mixture, but very popular. A large schnapps glass of raspberry juice is poured into the Weissbier. If you’ve been marching for a couple of hours and are thirsty, anything tastes good.)
Die Grenzboten: 1892, Volume 51, Part 2As early as 1850 the Magdeburgische Zeitung is carrying ads for “raspberry lemonade extract”, which mixed with three or four parts water would give a “most refreshing lemonade”.
It wouldn’t surprise me in the least to find out that people were pouring the stuff in their beer even then.