It worked! These clear marbles came with beads I got at an estate sale. So, they were basically free and I didn't care if they got ruined.
I wondered, then, after they were shattered, if the cracks could pick up color. So I rolled them around in food coloring, and HEY LOOK! It worked!
Thanks for the great idea!!! I bet the color would be more brilliant if I'd let them sit longer, but this is after about two minutes. I didn't want the food coloring to dry and require scrubbing.