  We investigate the magnetic field dependent photo-physics of individual
Nitrogen-Vacancy (NV) color centers in diamond under cryogenic conditions. At
distinct magnetic fields, we observe significant reductions in the NV
photoluminescence rate, which indicate a marked decrease in the optical readout
efficiency of the NV's ground state spin. We assign these dips to excited state
level anti-crossings, which occur at magnetic fields that strongly depend on
the effective, local strain environment of the NV center. Our results offer new
insights into the structure of the NVs' excited states and a new tool for their
effective characterization. Using this tool, we observe strong indications for
strain-dependent variations of the NV's orbital g-factor, obtain new insights
into NV charge state dynamics, and draw important conclusions regarding the
applicability of NV centers for low-temperature quantum sensing.
