Colour–colour diagram of the western lobe showing that the spectral shape of the plasma located in different regions follows different radiative model curves. Pixel-based values are shown as circles (blue shows the inner lobe, red the outer lobe, and yellow the middle region of the lobe). Black diamonds and squares are the values computed by integrating the flux density in the regions shown in the bottom left panel of Fig. 8 for the inner and outer lobe, respectively. The grey solid line represents = . The other lines indicate the spectral index evolution of ageing particles for various models. In particular, the black dashed line represents a CI model with α_inj = 0.45, and the black solid line represents a CIOFF model with α_inj = 0.45 B = 3 μG and t_on = 20 Myr. The blue dotted line represents a JP model with α_inj = 0.7, and the green dash-dotted line represents a JP model with α_inj = 0.57. The average error bar on the spectral indices is shown as a reference in the bottom right corner.