Infrared color–color diagram for the post-AGB star and PN candidates in our sample. Sources above the dotted (solid) line have a F_9   μm/F_J ratio larger than 1000 (100). The long arrow at the top right corner represents the shift in the position of a source on this plane after correcting it from a Galactic interstellar absorption of A_V = 10 mag The horizontal small arrows associated to some of the sources indicate that they are not detected in the J band and thus their location can still be shifted towards the right.