  We investigate the electron spin resonance of an ensemble of Nitrogen-Vacancy
(NV) color centers in a bulk diamond crystal. The four possible orientations of
the NV-center in the lattice lead to different dependences on the magnitude and
the orientation of an external static magnetic field. Experimental results
obtained with a continuous microwave excitation are in good agreement with
simulations. In addition, we observe that the average radiative lifetime of the
NV color center is also modified when the external magnetic field is applied.
This variation is explained by the mixing between mS = 0 and mS = $\pm$1 spin
states of the NV-center with different radiative lifetimes, due to magnetic
coupling. These results are of interest for a broad range of applications, such
as spin-resonance-based magnetometry with a high-density ensemble of NV-centers
