I am one of those KDE 3 die hards.
With KDE 3 I was able to set my wallpaper to an image, and then to add a solid color (with some alpha value) on top of it. I find it makes the background interesting but not destructing.
I can't find this feature in KDE 4, however.
Is it missing all together, or did the KDE devs just moved it to someplace I can't find?