In the case of 3C337, the colour-colour plot is different from that of Cygnus A, showing a kink. This could be evidence of two separate components in the spectrum, as the colour-colour plot is similar to those shown in Fig. 11. While this is an interesting possibility, it is clear from the images and spectral index cuts (Pedelty et al. 1989) that there is complicated structure around the eastern hotspot with a non-monotonic spectral index gradient, complicating the interpretation of the radio spectral shape.
The colour-colour plot for 3C289 is sparse, but is consistent with being similar to that of Cygnus A. What is interesting is that this source shows a spectral index asymmetry (Liu & Pooley 1991), with the more depolarized side having a steeper spectrum. There is some indication from the colour-colour plot that the spectra of the two sides of the source are different. This is a potential tool for the study of asymmetries in radio sources. If the spectral shape is different on the two sides of the source then it gives an opportunity to probe the physical conditions in the two lobes.
___________________________________ Peter Tribble, peter.tribble@gmail.com