By comparing photoluminescence and photo reflectance spectra taken on a series of InGaN-GaN quantum wells grown under identical conditions except the growth time of the InGaN layers, we could monitor the Quantum Confined Stark Effect (QCSE) without changing the nanotexture of the alloy layers. Our results indicate that, for quantum wells which radiate in the red, the contribution of the QCSE superimposes on the intrinsic localization phenomena of the carriers in the InGaN alloy. and is larger by one order of magnitude. Interpretation of data for samples that emit from the blue to the red can provide only partial conclusions if both localization effects and QCSE are not taken into consideration.