Color-color plot for the 12 962 sources in our MPS sample, where α_low is the spectral index between LoLSS (54 MHz) and LoTSS (144 MHz), and α_high is the spectral index between LoTSS and NVSS (1400 MHz). 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 0.18. The contours represent 17, 60, 168, and 308 sources. It should be noted that there might be PS that are not indicated in red because they are hidden by the contours. The red dashed line represents the 1:1 ratio of the spectral indices. The median error bars are plotted instead of the individual errors. 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 ± 0.2. Mock SEDs are shown, to indicate the rough shape the SED of a source in each quadrant would have.