Different sources for T_eff as compared to the SDSS-based temperature scale prepared in this work and taken as a reference (ΔT_eff = 0 K is marked with a dotted line), for all the 60 cluster RGs, labeled with their KIC ID. Orange circles refer to the original temperatures from the SDSS photometry (Pinsonneault et al. 2012), blue squares to (V − K) color temperatures (Hekker et al. 2011), green triangles to ASPCAP temperatures from APOKASC (Pinsonneault et al. 2014), and purple stars to spectroscopic temperatures from Molenda-Żakowicz et al. (2014). The 1σ uncertainties on each value are also overlaid. The dark gray shading around ΔT_eff = 0 delimits the 1σ uncertainty adopted on the reference SDSS-based temperature scale. The seven stars of NGC 6791 that show cooler SDSS temperatures (see Sect. 2.2) are marked by an asterisk.