I can't figure it out. And I'm sure this is not how he's doing it, but could you potentially use water mixed with very fine metal particles and then use a bunch of small electromagnets turned on and off quickly to pull the water/metal mixture along the path?
Agreed, it is likely for the camera. A lot of blue dyes for water fountains and such are copper sulfate. Dyes for tanks or fish ponds are increasingly methylene blue, food dye.
I thought I would just extend the poster's thought experiment, not imply that magnets were indeed used.
