First, you are not allowed to look to much sexy, so choose a midi dress or skirt and do not go sleeveless. A minimum cap sleeve is expected.
Wear a classic black or white pumps, avoid platform and sneakers.
If we are talking about color, than remember that each country has its own rules. For example, in Malta people tend to go for black. But, of, course, is acceptable to come to such ceremony in grey or white (not total white, just one item, for example blouse or pants).