Once she studies your hair, its condition, its style, its cut, and your lifestyle (e. g., how much time you plan on spending on your hair each day), she will then make some recommendations for color and maybe even a more suitable style for you. Plan on being there for a few hours.
Still if that sounds at all appealing you might first want to stop and ask yourself some questions. Or at least think about these points which can help you decide among the many hair color options. Since the goal is find out which will actually work out best for you and your hair.