Colour-colour diagrams for a typical FR II galaxy 3C 457 (left), a typical FR I galaxy 3C 465 (middle), and 3C 442A (right) in the ${\alpha }_{144}^{57}-{\alpha }_{1637}^{887}$ plane. The colour scale denotes the distance from the core relative to the approximate size in Table 2. The solid grey error bar denotes the possible systematic shift of data points due to calibration errors and the thin black error bars denote the rms flux uncertainty. The solid line denotes a pure power-law and the dashed line a JP model with α_inj = 0.78 ± 0.09 for 3C 457, α_inj = 0.5 ± 0.1 for 3C 465 and α_inj = 0.46 ± 0.09 for 3C 442A. The two lobes are differentiated by either showing a square or triangle. For both sources the region containing the core is excluded from the figure.