Various colour-colour diagrams constructed from our master-catalogue. The left-hand panels show data for the Orion A region, while the right-hand panels show the same combinations for the extinction free control field. Evidently, the Orion A field is heavily affected by reddening due to dust in the molecular cloud. The Spitzer photometry for the control fieldwas derived with a linear transformation from WISE data and therefore shows a larger dispersion when compared to the high-sensitivity data of the Spitzer Orion programme.