Stick to a mixture of cocktail attire and business casual when trying to pick out things to wear. A more conservative little black dress may be appropriate for an art opening. Also opt for a simple but elegant blouse paired with either a pair of nice slacks or a skirt.
Go for a more casual look by pairing a long-sleeve button-down shirt with a pair of tweed or wool slacks and a pair of strappy heels. Keep colors simple as to not upstage the artist at his opening.
Pull your hair back into an elegant bun or up-do. Or, leave your hair down but styled nicely. Keep makeup simple and wear only a couple pieces of jewelry so you're not too flashy. Carry a small purse or clutch that goes with your outfit.
Wear a suit and tie for a more formal look. If you don't want to be too formal, a neutral or pale-colored shirt with a nice jacket like a dark blazer, tailored suit jacket or tweed jacket.
Match your top with a pair of fitted wool slacks, or a nice pair of khakis or fitted dress pants. You want to look put together and not messy, so fit is key. If you need a belt, look for a simple leather belt in black or brown to go with your outfit.
Dress in dark-colored loafers or leather shoes with the rest of your business casual look. Add in a conservative watch as a stylish accessory. Finish your look by styling your hair in a comfortable way. Keep your facial hair well-groomed and neat.