[Zoner]

Here's the best explanation I have found for those foot pads:

"The footpads are loaded with chemical indicators (compounds that change colors when the general environment changes appropriately, i.e. reacting to a pH change or the change in concentration of a specific compound such as urea). Your body uses sweat as a method of removing urea - the reason sweat marks are sometimes yellow. So your feet sweat all day and night and the pad is loaded with an indicator that changes color to black when the pH becomes more acidic (urea is acidic)."