I put three layers of color on each shot glass, all different, then sanded with a superfine sandpaper.
Here's the lesson I learned: because the underglaze was only one coat of each color, I quickly, and TOO easily, sanded through all layers.
If you're going to do this, do a couple of coats for each color applied, letting dry thoroughly between each coat.
These will be bisqued this week, then I'll try to do a little more light sanding before I clear glaze and re-fire. We'll see how they turn out.