I see those machines at local health fairs (like, associated with runs, Marathons, etc) , along with holographic balance bracelets, etc. Unfortunately there's always a steady stream of customers.
Very simple experiments would demonstrate that they do nothing. Indeed, it's so obvious that the more sophisticated practitioners will admit that the color comes from the electrode and the salts, but still claim it's doing something. Most of the time though it's sold on the visual appeal of all those toxins coming out of your feet.
It's the placebo effect. Which of course DOES actually work (at least it seems to the patient it does). But there's no evidence that I know of it's actually changing anything in the body.
"detox" in general seems to be a bit of a scam buzzword.