BPP and createArrayBuffer changing colours

Thanks for your suggestions Gordon, I tried them all, but in the end redrawing the entire screen was too slow to look good. Your g.scroll and g.drawImages solution was the best, but moved the widgets as well which wasn't ideal.
I've abandoned the idea of a general function to wipe to a different screen, and opened https://github.com/espruino/BangleApps/p­ull/2547 with the much simpler solution of just clearing the screen and redrawing the text for this clock. I'll have to come up with some more imaginative animations for my other clocks!