Observed and de-reddened (BP–RP) color as a function of effective temperature. Only stars with spectral S/N > 30 are shown. The left panel shows the observed (BP–RP) color vs. T_eff, with the stars color-coded by Galactic latitude as a rough proxy for the expected dust reddening. The gray line delineates the MIST isochrones’ prediction for (BP–RP) color as a function of T_eff for ZAMS stars with initial [Fe/H] = 0. This panel illustrates that our sample stars in low Galactic latitudes suffer from a large reddening effect, leading to much redder colors than stars at high Galactic latitudes. The right panel shows (BP–RP)₀, the de-reddened color, based on the 3D reddening map of Green et al. (2019), with the stars sorted and color-coded by log g. The panel illustrates that such de-reddened colors qualitatively agree with the isochrones, but the significant spread in color at any given temperature implies that the photometric colors alone would be too imprecise to estimate T_eff robustly for stars above 10 000 K. The very hot stars with little reddening and very high log g are presumably near subdwarf B stars of modest luminosity (see Fig. 14).