The current calibration of the Ivezić et al. 2004 (2004AN....325..583I) define a "Principle Color" coefficients for HSC is based on the three RC2 tracts.  These numbers should be refined based on the full PDR1 reprocessing (see: https://confluence.lsstcorp.org/display/DM/S17B+HSC+PDR1+reprocessing).

This requires running the colorAnalysis.py pipe_analysis scripts on all the coadds from that processing, and note that we want to have this calibration performed using the per-object Galactic Extinction correction of DM-13519 (so that ticket branch should be used for pipe_analysis and obs_subaru if DM-13519 has not yet been merged to master at the time of running).