I've done samples using the same colors on top (once dry), a different color, flat cs and crumpled cs. They all turn out nice. I did one that is cracked glass over the top. It was awesome and everyone looks at it longer than other samples.
Here is one that has been uploaded already. It is done with markers, same colors both times. If I get a chance I'll try to upload the cracked glass one too. It was done with a different color over the top.
Gallery at Splitcoaststampers