I can't remember. The first time I saw it, it was using the afterstep or enlightenment progeam for win98. After that, I found that it was a registry key you had to add and then modify the value to change the color of the transparency. 7 was the number I used I think (Blue). There was also red, yellow, green, clear, dark and light versions of each. It was REALY cool, but I had to rebuild my 98box after a while and it had lost it's novelty and I forgot how to do it. I had a program about two weeks ago that would do it, but at the time I was running NT4 and didn't need it ) Now I do.... go figure.
If you can find the way to do it, it is pretty cool.