To do this directly during a render, use my "Artistic Lens".

See this thread for how to use the lens:
 
http://www.renderosity.com/mod/forumpro/showthread.php?thread_id=2754029

Then put this shader on the lens. This is "HSV GC" - a technique I made up. I got the idea from HSV ETM. I just changed the function to the GC function instead of the TM function, but kept the HSV idea that preserves hue and saturation.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)