The first colour photographs were created by ap rocess introduced by Edmond Becquerel in 1848. The nature of these photochromatic images colours motivated ad ebate between scientists during the XIX th century,w hich is still not settled. We present the results of chemical analysis (EDX, HAXPES and EXAFS) and morphology studies (SEM, STEM) aiming at explaining the optical properties of the photochromatic images (UV-visible spectroscopya nd low loss EELS). We rule out the two hypotheses (pigment and interferences) that have prevailed since 1848, respectively based on variations in the oxidation degree of the compound forming the sensitized layer and periodically spaced photolytic silver planes.Astudy of the silver nanoparticles dispersions contained in the coloured layers showed specific localizations and sizes distributions of the nanoparticles for each colour. These results allowustoformulate aplasmonic hypothesis on the origin of the photochromatic images colours.