Chelated iron will darken a KBG lawn without stimulating top growth, but its color affect is short term. If your soil is neutral to alkaline, sulfur can also darken KBG without stimulating top growth. Sulfur has a much longer term affect on color than iron. Finally, plant growth regulators applied to KBG can reduce the growth rate. Some of these regulators do so through cell size reduction (Primo Maxx) that helps to concentrate the green color into a smaller blade. Though regulators will not increase blade width, they can darken a KBG lawn, reduce the rate of top growth, stimulate root development and increase overall density.