Can you tell us a little about your plant, what are the growing conditions, light, and temperature? What are your watering habits like? Is this an established rooted plant or was it started from a cutting?
Are the leaves just slightly yellow or are they yellow enough that they are dropping from the pant? Leaves that are slightly yellow can be a lack of nutrients and often times 1Tbsp per gallon of Epsom salts to water will work to provide the plant with what it needs to produce more chlorophyll to green back up.

Mike