As a freelancer, I've worked for companies whose requirements have varied from the hat (bloody ridiculous requirement in a Citation!) and double breasted jacket to jeans and T-shirts.
The one company I worked full-time for, our normal uniform was blue trousers, white shirt with company tie, blue NATO style pullover if an individual wished; jacket was blue/grey IAT leather by Aviation Leathercraft. Silver stripes. For flights like base check, maint posit or air tests we could wear jeans or chinos and company-braded polo shirt.