Position of the sources included in our sample in a colour-colour diagram for post-AGB stars. The blue pentagons show the sources included in this study, the green squares represent water fountain sources observed by Khouri et al. (2021), and the grey circles correspond to other known post-AGB stars catalogued by Szczerba et al. (2007). The vertical arrows indicate our assessment that the J-band magnitude of GLMP 950 and GLMP 953 is dominated by contaminating sources close to their position on the sky (see Sects. 4.2 and 4.3 and Appendix C).