Color-color diagram of sources, including cross-matches and spectral index histograms. Here, we display sources with either (S/N₁₅₀ > 5, S/N₂₂₀ > 3 and S/N₂₈₀ > 1) or (S/N₁₅₀ > 1, S/N₂₂₀ > 3 and S/N₂₈₀ > 5). The populations of synchrotron (at left) and dusty (at right) sources are visibly separated in the 150-220 GHz spectral index. A vertical line at ${\alpha }_{150}^{220}$ = 1.0 shows our separation criterion. Counterparts from external catalogs are shown as circles. SPT has detected sources in both populations, and most synchrotron sources have counterparts in SUMSS, AT20G, and ROSAT; dusty sources are more likely to have only counterparts in SPT or IRAS. At top left and bottom right we show the histogram of ${\alpha }_{150}^{220}$ and ${\alpha }_{220}^{280}$, respectively. In the case of ${\alpha }_{150}^{220}$, both populations are clearly separated; we show the double Gaussian fit of separated components and the sum. In the case of ${\alpha }_{220}^{280}$, both populations are not clearly separated, and there is no clear local minima in this case.