Colour-colour plot of spectral indices for the radio AGN sample. The different quadrants are labelled with the spectral shapes of the sources lying in that region. The spectral shapes are also shown above the labels. The red square marks the region where flat-spectrum (FS) sources lie. A 1:1 line is shown in blue. While studying the link of spectral shape with gas kinematics, only HFPS, LFPS and NPS sources are considered.