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.
Oh don't give me none more of that Old Janx Spirit
No, don't you give me none more of that Old Janx Spirit
For my head will fly, my tongue will lie, my eyes will fry and I may die
Won't you pour me one more of that sinful Old Janx Spirit