H–K_s vs. J–H colour-colour diagram of the four observed fields in W51 showing the positions of all the stars in each field (grey points) and the sub-clusters (black points) defined by the circles in Fig. 2. The red triangles indicate the spectroscopically classified massive stars. The red triangle in panel b is the position of LS1. The red squares in panel d are the massive stars classified by Figuerêdo et al. (2008) and Barbosa et al. (2008). The blue dashed lines are the unreddened 1 Myr isochrone (around H–K_s = 0) and reddened with A_Ks = 3.0 mag (around H–K_s = 2.5 mag). The blue solid lines are the unreddened 1 Myr PMS isochrones and the reddened isochrone with A_Ks = 3.0 mag. The blue dashed-dotted is the reddening line connecting the high-mass end of the two isochrones. The red lines represent the lines at A_Ks = 0.45 mag.