Effect of the change in IGM transmission on the colour−colour (u−g,g−r) tracks of a galaxy with 2.5 < z < 3.5 and its selection. As an example we use a template galaxy with average properties at this redshift: 0.4 Gyr old, with E(B−V) = 0.1, a metallicity of Z_⊙/ 2.5, and an SFH timescale of 0.3 Gyr. The blue curve represents the (u−g,g−r) track for a low IGM transmission (mean −1.0σ), while the green curve represents the track computed with a high IGM transmission (mean +1.0σ). The red track is computed with the mean IGM transmission from M06. The grey dashed box corresponds to the LBG selection for this filter set (from Le Fèvre et al. 2013b). For each of the high- or low-transmission tracks the full line with squares (displayed at steps of Δz = 0.1) ranges from z = 2.7 to z = 3.5, while the dashed line is for z < 2.7 and z > 3.5. We use an apparent (observed) flux below the Lyman limit of 3% of the flux at 1500 Å as measured by Le Fèvre et al (2014) in the VUDS survey (see also Cooke et al. 2014). The black lines are the corresponding tracks computed with the M95 models.