All Calathea have dark undersides. This is an evolutionary characteristic that they developed while living on the forest floor of the rainforest, where only about 2% of the ambient light filters down through the canopy and understory. The colored pigments in the leaf undersides have specialized cells that allow the plant to pick up and photosynthesize 'green light', the wavelenth of light that bounces up off the forest floor. To increase the humidity adequately around your plant, especially in the dry winter months, a cheap cool mist vaporizer or humidifier set up in the shower to make a mist chamber would be very beneficial.
