Electrochromic display device has been fabricated using poly vinyl alcohol (PVA) films complexed with H3PO4 and H2SO4, as electrolyte and WO3 film as electrochromic material. The device response such as colouration and bleaching times are determined from the variation of transmitted light intensity measurement at 450 nm wavelength. Various device parameters such as colouration efficiency, transmissive contrast ratio and transmittance modulation have been evaluated and results are analysed. The complex electrolyte (H+/VA = 0.12) of PVA-H2SO4 shows maximum change in optical density (Delta OD = 0.13) due to its high conductivity value. It is observed that transmissive contrast ratio is dependent on acid concentration and increases with injected charge density.