By means of radiative transfer simulation we study the evolution of the far-infrared colours of protoplanetary discs undergoing inside-out dispersal, often referred to as transition discs. We show that a brightening of the mid and far-infrared emissi
on from these objects is a natural consequence of the removal of the inner disc. Our results can fully explain recent observations of transition discs in the Chamaleon and Lupus star forming regions from the Herschel Gould Belt Survey, which show a higher median for the 70?um (Herschel PACS 1) band of known transition objects compared with primordial discs. Our theoretical results hence support the suggestion that the 70?um band may be a powerful diagnostic for the identification of transition discs from photometry data, provided that the inner hole is larger than tens of AU, depending on spectral type. Furthermore we show that a comparison of photometry in the K , 12?um and 7u0?m bands to model tracks can provide a rough, but quick estimate of the inner hole size of these objects, provided their inclination is below ?85 degrees and the inner hole size is again larger than tens of AU.
