Photometric redshift constraints for all clusters (black points with 1σ errorbars reported in both panels) as determined with the simultaneous modeling of the red sequence (RS) and [3.6]–[4.5] color in Sect. 4. Red stars show the spectroscopic redshifts for SPT-CLJ2040 and SPT-CLJ0607, as well as the best current constraint for SPT-CLJ0459 (see Sect. 4). Bottom panel: adopted model RS m814–m140 color at M^* vs. redshift, for formation redshifts 2 <  z_f <  5 (orange lines). Light blue bars show the 1σ constraints from modeling only the RS color (dark blue assuming z_f = 3). Light green bars show constraints from the [3.6]–[4.5] color alone. Top panel: adopted model [3.6]–[4.5] color vs. redshift, for 2 <  z_f <  5 as in the bottom panel. The shaded area shows for reference the 16th–84th percentile range of [3.6]–[4.5] color vs. redshift for field galaxies in the magnitude range of our samples (m140 ≲ 23.5). Dark green (bottom panel) and white (top panel) bars show the impact on the best-fit redshift (from IRAC color alone and RS + IRAC color, respectively) of a ± 0.03 AB mag systematic offset on the [3.6]–[4.5] color.