That's fun to do, and it really makes your skin library so much more flexible. One other (coloring) thing that I have had fun playing with is the "lights effect" in Windows FX. You can get some amazing gradients that way.
It seems to me that scripting for Windows FX might one day make the gradient shifting possible, not only te way you envision it, but in other ways too... for instance the title bar could run colors like a marquee... Kind of psychedelic sounding, I know, but i think also it could be done subtly with some finesse to make a more soothing effect..
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