First of all, you need to ensure that the sunglasses have lightly tinted lenses. A dark shade will not only make you look stupid, but also affect your view.
If your makeup and dress compliment, you can also wear sunglasses with clear lenses. Wearing white lenses was considered to be a stupid act a few years ago, but it has become a fashion now.
Especially if you are wearing a casual dress, you can beautify your look with a cool pair of glasses. It may not look good on everybody though.
Any light shade of brown is highly recommended for night events. However, you need to make sure that the lenses match your clothes. How could someone look cool if he wears a green shade with black dress.
You can compromise a little on darker shade, if it compliments your entire outfit. However, it is not a good idea to get a dark pair of sunglasses just for the sake of wearing it.
If you have planned to wear sunglasses throughout the evening, you must let someone else drive. These glasses can hinder your vision, especially when there is a beam of cars coming from the opposite direction.
When buying sunglasses for night events, make sure the pair is not too sensitive. You may have to remove the sunglasses periodically, as is hardly good light on formal functions. Since you will keep the glasses in your pocket after removing, the sensitive ones may crack quickly.