The variation of the observed colour temperature and spectral index in two layer models (solid curves). The first layer is at fixed temperature of T = 10 K. The other layer is at 11 K, 12 K, or 14 K (black labels in the figure) and its relative mass is varied between 0.1 and 10.0 (four values are indicated on the curves). The intrinsic spectral index of both components is β = 2.0.