Colour–colour diagram of the SPIRE fluxes with the tracks of the local starburst galaxies M82 (blue) and Arp220 (red) over-plotted. The tracks represent the observed colours as expected from the same SEDs observed at different redshifts from z = 0 to z = 6 with redshift steps of Δz = 0.1 The bold points are the high-z candidates which lie in the same colour space than the evolved tracks at z >  2. The upper right area of the plot shows the average error bar for the high-z candidates. all these methods are based on MBB models, where the redshift evolution of the SEDs is not included. Therefore, more sophisticated SED templates should be used to investigate the correlation between redshift and colours.