I was surprised at how much my random guesses were correct, based on the style of the flags. (Mostly getting it wrong, but maybe 20% success rate)

There is definitely a clustering of design styles. British territories for example tend to have elements of the Union Jack incorporated. Germany, Luxembourg, Italy, that area tend to have more the simple 3 stripe designs.

I found myself occasionally guessing a right answer for flags in regions I knew very little about. I guess that sort of knowledge about design trends seeped in somehow.