NIR colour-colour diagram for our targets in M17. The colours and symbols are the same as in Fig. 2. The dashed lines represent the reddening lines for an O3 V and an M0 III star. For this we took the average Galactic value of R_V = 3.1. In Sect. 4 we show that R_V varies from 3.3 to 4.6. The solid line indicates the unreddened main sequence. Most confirmed mYSOs show evidence for the presence of a NIR excess.