ETA: at sunset, colors might not matter so much, so you could try things on at around that time, too, to see if you want to bother coordinating colors. If not, just go neutral because you can re-use neutral accessories.
If it’s going to be 80 and you like the look of your shoulders and arms, I’d go sweaterless. I’d also choose a nice, big belt in a contrasting or neutral color – black or red could be good, for instance – and if you go with a sweater, try it on with the belt. One really nice look I’ve seen is tucking the sweater under the belt to keep it in place and tie the outfit together. BUT I’d go with a more neutral sweater – black or grey perhaps – to keep from being too busy looking.
Jewelry – simple and classic. If you have nice pearls, wear those, otherwise I’d go with a simple necklace and earring set only if you normally wear them.
Shoes – simple and neutral. Black flats are always nice. Go with a pop of color if you can find one that coordinates well.
Colors – if you want to add some non-neutrals, I’d go with red, hot pink, yellow, orange, or purple. Choose solid color shoes and belt from these colors – I wouldn’t mix the red or purple with the other colors or each other, but yellow and orange should play well with everything else. Everything but the shoes and belt should be neutral though – black is always nice. Wear your dress to to the store to pick out/try on the accessories. If you add color, your outfit will look less formal than if you stick to neutrals.
Fiance – slacks and a nice button-down shirt should do. Depending on his coloring/preferences, khakis, a white/light blue shirt, and a dark blazer could be very snappy. Have him go with neutral shoes and belt. I wouldn’t be too matchy, so he doesn’t need to reflect your color choices, just not clash with them. He could pick up your neutral color in his choices, or the accent color you choose. He could go with a tie, too, if he likes them, but I tend to prefer the look of men in blazers without ties. If he’s really bold, purple and pink tend to look stunning on men, imo, even if they have to be convinced of that fact.