Radio colour–colour diagram with a comparison of the high-frequency radio spectral index α_high (1400–5000 MHz) with the low-frequency radio spectral index α_low (144–1400 MHz) taken from Table 2. The blue solid line indicates points of equal spectral index, indicating a power-law radio continuum spectrum. Most of the data points lie below the blue line indicating spectra with a convex curvature. Data points above the blue line indicate concave spectra, as may be explained by a higher thermal fraction at high frequencies.