The electrochromic changes of the polyaniline films during the switching from reduced to fully oxidated forms have been studied by the in situ measurement of the colour spectrum. Two-beam spectrometer is used during the protonation-deprotonation processes in electrochemical cell designed for this measurement. Colour transmission of different grade of polyaniline film layers changes were observed in different electrolyte solutions, temperature and applied voltage (e.g. redox state).