One could do a lot worse than learn from tutee's perceptive observation in an AskAndy discussion thread on shirt and tie matching. He argues that if your shirt and tie combination is lighter than the suit then the effect is more formal. On the other hand, if it is darker than the suit, the combination is "sportier".
This may seem counterintuitive but it works. Try to visualize a light or sky blue shirt against a navy blue chalkstripe for example. And then try on the ensemble to see for yourself.