Residuals of our HST–2MASS filter transformations with respect to the J − K _S color for our fitting sample (773 spectra of 648 unique stars). Color maps show relevant stellar parameters where available, including effective temperature T _eff, metallicity [Fe/H], surface gravity log(g), and reddening E(B − V). Reddenings are as described in Section 2.2.2; all other parameters are adopted as is from the literature sources.