Relation between stellar [2.25]–[2.34] color relative to the RGB mean color and EW(CO) for the early-type star candidates (green circles) and late-type giants as a reference (red circles). In the relative [2.25]–[2.34] color, the RGB stars and early-type stars are expected to have a color of [2.25]–[2.34] ≈ 0 and [2.25] − [2.34] > 0, respectively. The right-hand side axis shows the corresponding T_eff derived by Eq. (2). The blue arrows represents spectral type in the case of giants (according to the Table 2 in Meyer et al. 1998). The horizontal dashed line represents the upper limit of EW(CO) to determine T_eff with Eq. (2).