Color indices of the candidate counterparts of the highly absorbed X-ray sources of our sample. For the sources reported with a filled circle, the brightest optical and infrared candidate counterparts are coincident, while the sources reported as an open circle correspond to those with the “a” flag in Table 3: in this case, there is also a possible alternative counterpart, with a slightly brighter V magnitude, to the selected infrared counterpart. We also indicate the main-sequence stars and a bar which shows the reddening corresponding to A_V = 1 mag.