Color-color diagram of FXRTs 8, 9, and 14 and X-ray sources classified as stars according to Criterion 2 (see Sect. 2.5.2) with Pan-STARRS, DECam, and 2MASS counterparts (gray filled circles). The expected parameter space of stars with different ages (log(Age)  =  7.0–10.3), metallicities (from [Fe/H]  =   − 3.0–0.5), and attenuations (A_V  =  0.0–5.0) taken from the MIST package (Dotter 2016; Choi et al. 2016) is overplotted as an orange region.