Yellow-emitting InxGa1-xN/GaN multiple quantum wells (MQWs) with different pairs of In composition and QW thickness have been grown by metal-organic chemical vapor deposition on sapphire substrates. We show that a trade-off between the MQW crystalline quality and the quantum confined Stark effect has to be found to maximize the room temperature photoluminescence efficiency. With our growth conditions, an optimum design of the MQW is obtained for x=0.21 and a QW thickness of 3.6 nm.