The invention relates to a pad printing instrument comprising an ink supply system capable of control the temperature, viscosity and colorant concentration of an ink in an ink cup. Such controls are achieved by continuously adding and mixing a cold ink having a composition identical to the ink in the ink cup but having a lower temperature. The continuous addition of a small amount of a cold ink into the ink cup could compensate heat generated by the friction between the ink cup and a cliché and could minimize the evaporation of a diluent in the ink and change in the concentration of colorants in the ink and ink viscosity. The invention also relates to use of a pad printing instrument comprising an ink supply system of the invention for producing colored hydrogel or silicone hydrogel contact lenses.