A button down collar is considered to be less formal. As for what pastors wear, it depends greatly on the denomination and also on the circumstances. Many would wear "street" clothes. So, if the wedding was not kilted, what would you wear? Sport coat, suit, shirt and tie, sport coat and open collar shirt, etc. That could help you decide what would be analogous to the kilted wedding. To me though, the greater question is... ... what color hose to wear! <grin>