The FDP used in the 1950's red flags with a golden cross and the black eagle emblem with white letters 'FDP'. This was a time when black-white-red colours were prominently used by the FDP, for instance on election posters. The flag (...) incorporated black-red-gold as well as black-white-red colours, which led to serious controversies in the party committees. Obviously the flag was not even adopted by the party leadership. I cite from the protocols of the party leadership (session of 30th July 1953, my translation):
Rademacher: "After all, who has decided on this? And who has, by the way, adopted the federal party flag? (...) The most unbelievable things have happened in the party. They have given themselves a flag, which the highest committee, the party leadership, has not even discussed. I would like to know, I have to come home with an explanation: how did this flag come about?"
Rademacher did not get a definitive answer during this session. Obviously the flag as well as the decoration for the party convention at Bad Ems (1952), a very controversial convention, had been designed by somebody in the propaganda committee of the FDP.
By the way, there is some uncertainty regarding the precise design of the flag. Rabbow 1965 shows the flag with a normal centered cross. However, in a FDP brochure for the 1953 election campaign the cross is shown as a scandinavian cross.
These flags and this symbolism went out of use in the 1960's, according to Rabbow 1970. Since 1966 and even more since 1972 the FDP uses mainly the colours yellow and blue.
Sources: Rabbow 1970; Rabbow 1965; FDP-Bundesvorstand, Die Liberalen unter dem Vorsitz von Theodor Heuss und Franz Blücher; Sitzungsprotokolle 1949-1952, Droste Verlag, Düsseldorf, Brochure Wahlkampf und Werbung: Ein Leitfaden für die Wahlkampfleiter, Propagandisten und Redner der Partei, 1953; Photo of a demonstration in Hamburg in 2000, found in Internet; FDP website; Election posters at this website Marcus Schmöger, 31 March 2001