When it comes to formal events, matching the leathers of your belt and shoes is essential to looking well-dressed. There's no way around it: Brown goes with brown and black with black. Otherwise, you'll just look mismatched.
Patterns are great, but it is possible to have too much of a good thing. What has become known as "power clashing" — a style move where one wears more than one pattern in an attempt to look garish on purpose — has no place in modern dress.
The bottom button looks awkward when buttoned, and it constricts the waist, giving it an odd corset-style look. Leaving it undone does more than just give you a casual, laid-back vibe. It makes you look like you know what you're doing.