Amorphous and crystalline WO3 films show electrochromic properties by means of double injection of cations and electrons. At low substrate temperature thermal evaporation technique produces an amorphous material mixed to a nanocrystalline phase. When this material is annealed at temperatures higher than 400 degrees C a fully crystalline structure is obtained. Different absorption mechanisms are responsible for colored amorphous and crystalline WO3 layers, small polaron transitions and Drude-like free electron scattering, respectively. Furthermore, both O/W ratio and crystalline structure affect electrochromic efficiency of a WO3 film. It has been confirmed that a hydrogen tungsten bronze structure appears when the material is colored. Finally, a self-bleaching process occurs when a colored WO3 film is exposed to an oxidative environment.