If you want to get more mileage out of your separates by mixing and matching pieces, add a belt to pull it all together, like Kristin Davis does, here. Your belt color doesn't need to match your bag, shoes or separates, but the look is most cohesive if your belt goes with (either matches or contrasts with) one major element of your look. For example, if your separates consist of a white blouse, black skirt and red jacket, the easiest belt color to choose would be black because it matches the skirt.
Shop for a black belt