take 3 images, edit in darktable as monochrome, export all to tifs, gnu imp import all 3 as layers, color > compose using the layers as RGB, badaboom. an image where everything still is monochrome and motion shows up as color.
i took the 3 images using the pentax q's interval shooting mode at 1 second intervals so it would be perfectly still between shots. unfortunately it can only clear the buffer fast enough to take 4 images at that speed before giving up > . >
god i wish i wasnt so hardware limited.
on a nicer camera one could do a burst shot and get some very neat motion across the frames. you could go up to 6 frames if you color the extra 3 as CMY, could do 9 if you let every third be pure luminance. lots to work with depending on the kind of motion you want to capture and how busy you want it to be