Radio colour–colour plot showing the low-frequency (150 MHz –324 MHz) spectral slope on the x-axis and the high-frequency (324 MHz–1.4 GHz) spectral slope on the y-axis. The dashed line indicates α_low =−0.25, the cutoff above which we define sources to have extremely flat low-frequency spectra. Sources above this threshold are marked with squares in the plots throughout this paper. Sources are numbered with the IDs assigned in this study, as detailed in Table 1.