Which convention? Shoes I don't care, sunglasses as long as not the reflective type I don't really mind. Trousers or shorts, well if I am umpiring with a female colleague and she is wearing a skirt and I have trousers then really what's the difference? Shirts - oh yes as similar as possible please. I make the distinction based on the idea that I want to appear as a member of a team with my colleague and the players recognize us as such. The idea of uniform is to present that conformity at first sight. Those who say it's down to decision making and not appearance are then correct because however you look if you have a bad day it's a bad day, but I just feel being properly turned out is part of the ritual of preparation which makes a good day a bit more likely.