Radio colour-colour diagram for the 9768 LoTSS sources left in our Master sample. α_low represents the spectral index between the LoLSS (54 MHz) and LoTSS (144 MHz) flux density points. α_high represents the spectral index between the LoTSS and NVSS flux density points. To better illustrate the large number of sources around the median of α_low and α_high, given by (α_low, α_high) =(−0.6 ± 0.2, −0.8 ± 0.1), contours and a density map are plotted in this region. The contour levels represent 15, 75, 145, and 265 sources. The colours in the underlying density map show the number of sources for each pixel. The number of sources corresponding to each corresponding shading are illustrated by the colour bar at the top left of the plot. The grey lines show spectral indices of zero to illustrate the four quadrants of the plot. The dashed red line represents a one-to-one relation between α_low and α_high. The blue line at α_low = 0.1 illustrates the selection limit used to identify PS sources in this study. In the corner of each quadrant the shape of a typical spectrum in that quadrant is shown in grey. To avoid confusion, individual error bars are not plotted, but in the top left of the plot the median error bar size is shown. The histograms at the top and right of the diagram illustrate the distributions of α_low and α_high, respectively. These distributions have been normalised to the maximum value in the distribution. In these histograms Gaussian fits to these distributions are overplotted in red, and the dashed black lines show the median values for these distributions.