Well, who knows which
flag of Germany Data was commenting on? The colors would have been black, white and red for half a century (plus the same colors yet not in tricolor layout for an infamous decade later on). Perhaps they went to black, yellow and red at some point. Say, in the late 1980s or early 1990s when the Germanies reunited in the Trek universe... Or then at some point just before WWIII.
Our only limitation there would appear to be "One Small Step" where Ares IV
sports the familiar flag in the 2030s. Although really being able to tell takes some effort: