The continuous shift in photoluminescence (PL) spectra of porous silicon (PS) has been analysed for the samples whose PL peak wavelengths are photo-assistedly tuned from red to blue. The PS layers are formed by anodizing p-type (100) Si wafers in an ethanoic HF solution in the dark. The PL peak energy is controlled by in-situ post-anodization illumination by a tungsten lamp. The evolution of PL spectra with increasing the illumination time is studied in relation to that of PL excitation spectra, polarization memory, and surface chemical composition. The experimental results indicate that the green and blue PL spectra are attained only by photochemical etching without the growth of the surface oxide, and that the PL emission from PS is based on the oxide-free mechanism even in the blue band.