Nudes would be fine as i doubt you'll even see them under a long dress really. Personally i think an open toe would look nicest with this, but if youre getting married in fall/winter the closed toe flats will be better suited
Beige or any white if you do not want your feet to be noticed. Or take the opposite tack, and wear ruby or cobalt or emerald colored shoes, and have fun with it!
Since the dress is flowy and your feet will actually show, I wouldn’t wear a closed toe shoe in general... and then not a nude flat. The 2 styles won’t look very good together. But if you want flats, I’d definitely get an open toe sandal, like in your dress pic. Or something like these: