The colour-colour diagrams presented in this paper for realistic spectra agree well with the colour-colour plot for Cygnus A presented by Katz-Stone et al. (1993). This shows two things. First, that the model spectra presented here and in Paper I do indeed give a good description of the radio spectra in real sources. And secondly, that the radio spectrum of Cygnus A is described by a realistic ageing model. Katz-Stone et al. argued that their inability to fit the Cygnus A spectrum with standard models raised doubts about the ageing interpretation of radio spectra. While there may still be some problems with low-frequency curvature, the success of the models presented here strongly supports the standard ageing interpretation.
The model colour-colour plots presented in this paper are all of a very similar shape. We therefore expect there not to be too much variation in observed colour-colour diagrams. Two systematic effects have been identified. Increasing the importance of inverse Compton losses or diffusion moves the colour-colour locus down in the diagram, while changing the distribution of magnetic field strength can move the ``parallel'' section of the colour-colour locus relative to the diagonal.
The shape of the colour-colour locus depends on the magnetic field model. There is an indication that the field in Cygnus A is skewed, with rare regions of strong field on a predominantly low-field background. With no diffusion, the field strength in Cygnus A is about 8.7. A more likely interpretation is that this represents a lower bound on the field strength, and that electrons diffuse slowly between regions of different field strength, with a mean free path of 10 pc or less.