Panel a: [5.8] – [8.0] vs. [3.6] – [4.5] diagram used to identify different classes of YSOs based on the disk and envelope models of low mass YSOs as shown in Allen et al. (2004) and Megeath et al. (2004). Class II YSOs are expected to be concentrated in the small box, while Class I YSOs in the bigger box. Panel b: Same colour-colour diagram as (a) with the region shown by black polygon where Stage I YSOs are expected to lie (Robitaille et al. 2006). Panel c: [8.0] – [24] vs. [24] diagram showing the criteria used by YHA09 to choose their sample of possible YSO candidates (region to the right of the dashed line). The black arrows in (a) and (b) represent the extinction vector estimated for A_K = 2 mag (typical of the CMZ) using the relations from Nishiyama et al. (2009). In each diagram, the spectroscopically identified YSOs are shown using blue star symbols and the cool late-type stars using black filled circles. Other symbols and colours represent the classification of the SIMBAD matches to our sources by searching within 2.̋0 (see Sect. 3.1).