Sugar plays many roles in yeast breads, including browning. The most important role of sugar in baking yeast breads is to provide "food" for the yeast. Yeast breads without sugar will not rise properly, if at all. The amount of sugar called for in non-sweet yeast breads should not cause a problem for most diabetics. If, however, you still wish to use a non-caloric sugar substitute instead of sugar, use a recipe that has been specifically developed for the specific sugar substitute. The crust color most likely will be light.