Colour–colour diagram of the photometrically reliable catalogue: flux density ratio S₃₀₀₀/S₈₅₇ versus S₃₅₃/S₈₅₇. The red lines show the domain of the single modified blackbody emission models with fixed values of β. The blue lines show the domain of the single modified blackbody emission models for fixed values of the temperature. The locus β = 2 (red solid line) appears to be insufficient to fit all the observational data points (black dots) of the C3PO photometrically reliable catalogue.