Relation between the Gaia spectral index I_B and the B − V color. The upper x-axis relates spectral types of main-sequence stars to B − V (M. J. Pecaut & E. E. Mamajek 2013). Gray points: all 2527 stars with Gaia spectra, B − V colors, and no saturation issues; black, yellow, and gold points: 1195 stars with d ≤ 80 pc. The cyan curve is the best-fitting polynomial to the 80 pc data (Equations (4)–(5)). The clear correlation between I_B and B − V enables spectral classification with an accuracy of ±0.5–0.75 subclass.