  In order for Wide-Field Infrared Survey Telescope (WFIRST) and other Stage IV
dark energy experiments (e.g., Large Synoptic Survey Telescope; LSST, and
Euclid) to infer cosmological parameters not limited by systematic errors,
accurate redshift measurements are needed. This accuracy can be met by using
spectroscopic subsamples to calibrate the photometric redshifts for the full
sample. In this work we employ the Self Organizing Map (SOM) spectroscopic
sampling technique, to find the minimal number of spectra required for the
WFIRST weak lensing calibration. We use galaxies from the Cosmic Assembly
Near-infrared Deep Extragalactic Legacy Survey (CANDELS) to build the
LSST+WFIRST lensing analog sample of ~36 k objects and train the LSST+WFIRST
SOM. We find that 26% of the WFIRST lensing sample consists of sources fainter
than the Euclid depth in the optical, 91% of which live in color cells already
occupied by brighter galaxies. We demonstrate the similarity between faint and
bright galaxies as well as the feasibility of redshift measurements at
different brightness levels. Our results suggest that the spectroscopic sample
acquired for calibration to the Euclid depth is sufficient for calibrating the
majority of the WFIRST color-space. For the spectroscopic sample to fully
represent the synthetic color-space of WFIRST, we recommend obtaining
additional spectroscopy of ~0.2-1.2 k new sources in cells occupied by mostly
faint galaxies. We argue that either the small area of the CANDELS fields and
the small overall sample size or the large photometric errors might be the
reason for no/less bright galaxies mapped to these cells. Acquiring the spectra
of these sources will confirm the above findings and will enable the
comprehensive calibration of the WFIRST color-redshift relation.
