It took me a few attempts before I finally got the layered look. My husband, the scientist, watched me fail and then boasted that he does sugar gradients in the lab all the time, so he'd show me how it was done.
I laughed pretty hard when he made a purple drink just like I had done. After looking at two different sources, I found a couple tricks that work well. First, it helps to pour slowly, and second each layer should be poured directly onto ice.
The sugar content of each layer is critical. Don't forget to pour the beverage with the highest sugar content first, and then end with a sugar-free drink.