From an earlier answer of mine

The choice of the Nazi color scheme in their banners was wrapped up in the politics of the German national flag.

The flag of the united Germany in 1871 was the black-white-red tricolor with a more complicated war flag used by some military units such as ships. The imperial tricolor was something of an afterthought for the unified Germany; it originated during the earlier North German Confederation to fulfill a need for a unified ensign for German ships. The flag itself was neither a Hohenzollern nor a Prussian one, although it did incorporate some of their traditional colors. Yet the flag was indelibly associated with Imperial Germany and its lost war. A number of Germans in November 1918 instead opted for the black-red-gold tricolor. These were the colors of the German volunteers in the war against Napoleon and emerged in the post-1815 period as the colors that symbolized a united Germany. The Paulskirche parliament and other 1848ers tended to use the black-red-gold over the red flag as the symbol of their movement, seen here quite vividly in the Germania portrait that presided over the Paulskirche parliament as seen in this lithograph. The failure of the 1848 Revolution in Germany though was not the end of black-red-gold and after 1918, it became the new national flag of Germany. Not only was black-red-gold a break from Prussia and the Hohenzollerns, it was also reaching back to a past German democratic tradition. The SPD, Ebert in particular, invested a degree of effort into making 1848 and its black-red-gold as a historical origin for the Weimar Republic. The red flag did not disappear from the Weimar SPD, but was often coeval with the black-red-gold, such as in this 1920 election SPD poster.

Black-red-gold though was not an especially popular flag though among the German public, especially among the German right. Most of the German conservatives associated the black-red-gold with betrayal of the front in November 1918 and rebellion. They instead pushed for a restoration of the old imperial flag and some of the far-right Freikorps used the battle flag openly. The NSDAP was also among this far-right, but also possessed a degree of marketing savvy that stood it above its contemporaries. The NSDAP ensigns used the black-white-red color scheme of the imperial flag, but within a new set of symbols like the swastika. In this way the NSDAP flags bridged the divide between the various right-wing political symbols by repackaging something old-the imperial color scheme- into a new arrangement.

The Nazis once in power though did little to officially sanction the swastika flag as the new German national flag. While the black-red-gold was out, both the imperial flag and the swastika flag became dual flags of the new German state. For example, this piece of pedagogic propaganda has Germany with both the imperial flag and the swastika one. The general consensus was that the imperial flag stood for the state and the swastika banner for the ruling party of the state. This division led to various incidents in which exiles and other opponents to Nazism desecrated the swastika flags on visiting German ships on the grounds it was not desecrating the national flag of a sovereign state.

The Nazi government did not initially have too much of a plan for the flag. Bringing back the old imperial colors did bring in some support among the non-Nazi right-wing and various nationalists such as the German President Paul von Hindenburg. But the bifurcated symbolism did not suit the pretensions of the NSDAP, which desired hegemony over German national symbols. Von Hindenburg's death along with international protests against the swastika flag led to the official adaptation of the swastika flag as the sole flag of Germany in 1935.