Hi Lex,
Structure Detection should be used if you want to calibrate using stars on your image - in other words, if you do not have galaxy. With galaxy, Structure Detection should be switched off and only brightness limits should be used to select as much of the galaxy as possible. General goal is to have widest possible population of stars as your white reference (galaxy is ideal object from that point of view, but not always available).
The Color Calibration module works in the way it tries to grant sampled population of stars widest possible range of colors from red to blue, assuming there are stars of all types, from red giants to hot blue stars.
For more info about Structure Detection yes or no, I suggest Harry Page video on the subject. It is discussed there.
regards, Zbynek
