Integrated spectral index measurements. Top: tail spectral index versus galaxy disc spectral index. When emission is detected in three or more frequency bands, the spectral index and its error are determined from orthogonal distance regression fits. When emission is only detected in two frequency bands, the spectral index and its error are determined via the direct method (Eq. (1)). For galaxies where emission in the tail is only detected at one frequency (see Table 2), α_tail is shown as a 3σ upper limit. Bottom: radio colour-colour diagram for the galaxy discs. The high-frequency spectral index (y-axis) is measured between 700 MHz and 1.5 GHz, and the low-frequency spectral index (x-axis) is measured between 144 and 400 MHz. In both the top and bottom panels, the dashed lines correspond to the one-to-one relation.