Andrea: Colourlike doesn't make a new histogram every 1000 frames when you set every=1000. It samples the movie in 1000-frame-steps and still makes one histogram. All the every argument does is change the sample size.
My script generates a new histogram for every frame. That's the difference.
You should see results immediately with my script because it's not doing a WriteHistogram for the entire movie at one time, just the frame it's currently on.