Do you really mean an evening formal event? That means a dinner suit. Formal = black tie events, or white tie events...everything else is strictly informal. If I was to hazard a guess, I'd say you're attending an informal dress event. In which case, you can wear absolutely any colour tie with a navy suit, that's the beauty of the suit, it's the most versatile colour. Avoid black and grey ties, go for colourful ties, eg shades of red, shades of blue, shades of yellow, etc...