Colour–colour plot of spectral indices for the central region of the 129 radio AGN. x-axis shows derived using LoTSS and FIRST peak flux densities and y-axis shows derived using FIRST and VLASS peak flux densities. Regions in the plot for peaked, non-peaked and convex spectrum sources are marked with blue, orange and grey colour respectively. The spectral shape of a source in each quadrant is drawn at the corners. The error bar shows the typical error on the spectral indices. The hatched design marks the area which we classify as flat-spectrum region. Sources lying in this region are not included in peaked, non-peaked or convex spectrum groups. [O III] detections are marked with filled circles, and non-detections are marked with hollow circles.