Some of the manufacturers are apparently not claiming that the baths are as good as pixie dust for curing everything, and are even saying that the water color doesn't mean anything.
Ananlysis of the water indicated that a high concentration of iron was observed, consistent with the use of ferrous electrodes, but no organic molecules which, if present, would have indicated that the device actually worked. In a nutshell they are electrolysis units, capable of turning water into nasty looking dark brown water.
So why pay for sticking your feet in rusty water? If you do get suckered into paying for a session, make sure that you wash your feet off afterwards, my wife got headaches after a session given by her chiropractor.