The two colour diagram for the continuum of the Seyfert galaxies has been calculated with a two component model; a stellar component identical to that of the giant elliptical galaxies and the radiation of a gaseous component excited by thermal collisions. It is found that the continuum of several of the Seyfert galaxies (NGC 4151, 3C 120 and NGC 3516) fall in this region of the diagram. For NGC 4151 the observed continuum variations would indicate that the emission is purely thermal.