Multicolour photochromism of Ag-TiO2 nanocomposite material was found several years ago but its microscopic origin is still ambiguous. We study the initial stages of the effect of light exposure on extinction spectrum of this material prepared by photocatalytic technique. We propose a simple model to analyse the experimental data that suggests that the extinction changes are due to a pure shift of the plasmon resonance of silver nanoparticles after light exposure. The homogeneous spectral widths (100-200 meV) of plasmon resonance of individual classes of nanoparticles were found to increase with the resonance frequency.