Spitzer-IRAC versus CFHT WIRCam colour-colour diagram, with a track drawn from Bruzual & Charlot (2003) for a 1.4 Gyr simple stellar population, numbered by redshift. The horizontal and vertical dashed lines indicate the Papovich (2008) and Franx et al. (2003) criteria, respectively, representing colours of z  ≃  1.3 and z  ≃  2 galaxies (cf. the labels on the model curve). A redshift around z  =  1.6–2.6 is indicated for the majority of the ALMA galaxies. Arrows represent 2σ limits for the sources not detected in any channel or band.