Radio color–color diagram of spectral indices α_LV and α_FV. Blue points show sources detected in all three radio bands (LoTSS, FIRST, and VLASS), while gray symbols indicate sources detected in only two bands, for which upper limits are shown for the nondetected band. Upward arrows represent upper limits on α_LV for LoTSS nondetections, and rightward arrows represent upper limits on α_FV for FIRST nondetections, assuming representative flux-density limits of 0.35 mJy (LoTSS) and 0.75 mJy (FIRST). The dashed line indicates α_LV = α_FV, corresponding to a single power-law spectrum. The dotted lines mark α = 0 and α = 0.7 for reference. The blue- and orange-shaded regions highlight sources with significantly curved radio spectra, defined as Δα < −0.3 (convex) and Δα > 0.3 (concave), respectively. The fractions of sources consistent with power-law (∣Δα∣ < 0.3), convex (Δα > 0.3), and concave (Δα < −0.3) spectra are computed using only the three-band detections and are shown in the bottom-right corner. For clarity, only a random subset of 100 sources is shown for each class of upper limits, corresponding to ∼3% (LoTSS) and ∼7% (FIRST) of the full sample.