Black and white: The black-white has a strong connection to Prussia's history (Prussian flag here: [link]). The Teutonic Knights already had a white shield with a black cross. The first few lines of the Preußenlied (The Prussian Song, the national anthem of Prussia) are: "I am a Prussian , do you know my colours? The flag floats black and white before Me That for freedom's sake my fathers died To that know it hint my colours."
White and red: The red-white combination are very popular heraldic colors in the German-speaking areas and it's neighbor countries and in Europe in general (examples: Denmark, Poland, Switzerland, Austria, England, etc. have white-red flags and th Netherlands, France, Norway, Italy, Latvia, Island etc. have white-red in their flag as well). Furthermore the city arms of most German Hanseatic cities have a red-white flag or red-white-black flag such as Hamburg and Cologne (here: [link])