The light diffusion of hand lay-up glass fiber-reinforced polymer (GFRP) laminates was investigated with a spectrophotometric set-up using an integrating sphere. Reinforcement weights of the laminates ranged from 410 to 3280 g/m2 and fiber volume fractions were between 0.20 to 0.35. The influence of resin gel times on light diffusion was also studied. Experimental results showed that blue light was more diffused than other colors, particularly in laminates with longer gel times. The refractive indices of resin and glass fibers were investigated and a significant mismatch below 500-nm wavelength was observed. Numerical ray-tracing simulations were performed and the trend of the experimental total and diffuse transmittances were successfully modeled. These analyses confirmed that the spectral mismatch of refractive indices was responsible for the wavelength dependency of light diffusion in GFRP laminates.