Choosing useful pieces at a clothing swap

A friend has invited me to a small gallery opening, and the invitation gives no useful clues about what to wear. The clothes I already feel good in are black trousers and a plain cream shirt, so I want to start there. Something a little more expressive would be nice, provided it still feels like clothes I would choose on an ordinary day. To understand the color choices better, I have been reading about color analysis although assigning myself a season matters less to me than finding one accent that fits the overall outfit. Would you add a muted blue scarf to these pieces, or let a textured bag provide the extra interest? There is a short walk involved, and I would like to keep my comfortable shoes and bring an extra evening layer. If you could change just one accessory, what would help this outfit feel considered and still comfortable for the evening?