Hi Jastaflieger,
One very common flag of the Ottoman Empire, and of modern Turkey, was the white crescent and star on a red field.
https://en.wikipedia.org/wiki/Flags_...Ottoman_Empire
When it entered the Great War, the aircraft were eventually marked with the black squares. The usual assumption is that (A) the army of the Ottoman Empire, being Muslim, didn't really want to be flying aircraft marked with a Christian Cross. (B) They chose a white-outlined black square because these colors were in keeping with the black and white markings of Germany - using a red square with the white crescent and star could have resulted in being mistaken for Allied aircraft. In the early days, remember, the RNAS used a red roundel with a white center.
It was relatively easy to simply paint over the German Iron Cross and white field with the black square, leaving a white outline. There is little doubt that the squares were black. Hans Buddecke, in his letters, described his Fokker Eindecker as "my yellow bird with the black threatening eyes on the wingtips."
After the war the Turks returned to using a red square with white crescent and star.