Radio colour-colour diagrams. Data points are coloured based on the sampling (beam-size) region identifier as in Fig. 7. The black dashed line (one-to-one line) indicates a power-law with α = Γ. The solid lines are the theoretical KP (brown), JP (blue), TJP (red), CIOFF (cyan, purple, green for t_off = 10, 15, 20 Myr, respectively) ageing curves obtained with Γ = 0.55 and B₀ = 2.4 μG. The direction (eastward or westward) of the sampling with respect to region ‘0’ (centred on the radio core), frequency pairs, and resolution are indicated on top of each panel.