Thin films of WO3 are cast from quantum size colloids onto an optically transparent electrode. These thin particulate films are found to exhibit reversible electrochromic and photoelectrochromic effects. A blue coloration of the WO3 particulate film is seen when electrons are injected into the conduction band of WO3 by electrochemical or UV-excitation methods. The onset potential of the electrochromic effect is dependent on the pH and corresponds to the flat band potential of WO3. Spectroelectrochemical and microwave absorption experiments suggest that trapped electrons are the major species responsible for the blue coloration of the WO3 particulate film.