The most popular finish that is being used these days is a dark bronze. Sometimes called oil rubbed bronze or antique bronze. A newer trend ( or some would say old ) is a shiny brass finish.
If it is not a new house you are buying for then look to the finish colors of other hardware in your house to point the way. Things like cabinet pulls and for knobs.
If it is a new house and you don't know what any of the finishes will be, then considered the design and style of the light fixtures before the finish color.