Move the plant to your sunniest window. A loss of sunlight can cause leaves to lose their bright color.
Keep the temperature in the room between 60 and 80 degrees Fahrenheit. Also keep the plant away from any cold drafts.
Reduce watering during the winter. Let the soil dry out completely between waterings. Stop fertilizing as well. Only resume fertilizing when the new growth appears in the spring.
Mist the leaves with distilled water in a spray bottle daily. This keeps the humidity level up and prevents leaf drop.