Do you mean it's one of those kits that has you add sugar as one of the primary fermentables? Why do they do that???
A great substitute is dry malt extract (DME). It'll taste a lot better than sugar and will add color depending on which color you use (light, medium, dark).
It'll improve your beer a lot. The next step to make your beer *much* better is to add some adjunct grains just for flavor, like crystal or chocolate malt. You don't need to mash them and they make a world of difference with minimal effort.