The basic concept of "ionic foot bath" is that you immerse your bare feet in a basin of tepid saline water.
Electrodes are then placed in the water with (but not touching) your feet.
The process is supposed to "magnetically" and "ionically" draw "toxins" from the pores of your feet. These "toxins" and "heavy metals" then color the water, and you can visibly see these things leaving your body.
An alternative explanation is that water is colored as a result of chemical reaction supported by electrodes. Electroedes made of metal may react with saline water and may color the water even if your feet are not in the water.