Color−magnitude (left) and color−color (right) diagrams for the sample of 53 luminous YSGs in M 33. Infrared photometry is taken from 2MASS and from Thompson et al. (2009). Optical spectroscopy with GTC was obtained for the five dust-enshrouded YSGs shown with red squares, following the designation in Table A.1. Stars 6 and 13, which are flagged as candidate YHGs in the current work, are also plotted (large blue circles) using photometry from WISE at 3.4 and 4.6 μm. The J−K_s boundaries for YSGs are indicated by the two solid lines while the dashed lines correspond to the boundaries corrected for the average visual extinction A_V = 0.4 mag of M 33. We show the location on the diagrams of the known YHGs ρ Cas (green diamond) in the Galaxy and Var A (purple triangle) in M 33, as well as of the candidate YHG/LBV B 324 (cyan star) in M 33. Photometry for ρ Cas is taken from WISE at 3.4 and 4.6 μm, corresponding to upper limits due to the brightness of the object. Conversion to M_[3.6] is done using distance moduli of 24.92 mag for M 33 (Bonanos et al. 2006) and 12.5 mag for ρ Cas (Zsoldos & Percy 1991).