I'm not sure wearing a pocket square to an interview is a good idea, unless it's for a creative position/in fashion. It's always better to err on the side of caution for more formal positions. You don't want to miss out on an opportunity because you wanted to wear a pocket square or burgundy shoes. Keep it simple: Solid navy/charcoal suit, white/light blue shirt, neat tie and black (UK) or dark brown (US) cap toe oxfords. You'll have plenty of time to personalise your outfits once you have been admitted/have the job.