Color-color plot for the 108 473 sources in our GPS sample, where α_low is the spectral index between LoTSS (144 MHz) and NVSS (1400 MHz), and α_high is the spectral index between NVSS and VLASS (3 GHz). PS sources exist in the lower right quadrant and are indicated in red. The blue line represents the median of the error of α_low at 0.10, which corresponds to the median of the selection limit of PS sources. The contours represent 134, 416, 1349, and 2800 sources. The red dashed line represents the 1:1 ratio of the spectral indices. The median error bars are plotted instead of the individual errors, for readability. The normalized distributions of α_low and α_high are plotted, with a median and standard deviation of −0.6 ± 0.4 respectively −0.7 ± 0.5. Mock SEDs are shown, to indicate the rough shape the SED of a source in each quadrant would have.