Poinsettia leaves usually turn yellow as a result of A mineral deficiency such as of A lack of magnesium or molybdenum.
Over watering or not enough water could also cause the Poinsettia Leaves to yellow so make sure you're not over watering them or make sure you're giving them enough water.
Over fertilization is also another cause of Poinsettia Leaves turning yellow as well as Root Rot which can be caused by too much water.