I can't say I have, but I've mixed thicker colors in bags before, and, in doing that, I've often had the colors blend more than I would like as they were pushed through the tip. I imagine blending would happen to a greater extent with a looser icing too. I prefer to marble by piping patterns directly on the flood icing and then pulling a skewer through both icings to create patterns. I think those results are generally crisper and more precise (than ones achieved with dipping), but that's just my preference. I suppose the method one chooses all depends on the look one wants (more precise: pipe on accent colors and marble with skewer; softer and more ethereal: marble icing and dip).