It is black tie season once again and one way to add a discreet touch to evening clothes is to wear a pocket square that reflects a color in the evening's dress set. In the photo, a maroon square complements the stone in the visible stud.
Solid squares are safest but patterned squares may be considered. I have been of two minds about them for evening wear over the years however Britain's Prince Charles often wears a patterned silk in the breast pocket of his dinner jacket. If one were to follow in those royal footsteps, a black and gold silk with a white ground would complement onyx on gold studs, for example.
Buttons on the shirt front of course invalidate this particular trick, but they mean only that a colored square is unsupported on the shirt front. That is hardly a problem. After all, a man can always wear white silk twill.