The stability of the optical flux variation gradient for 3C120
  New $B$- and $V$-band monitoring in 2014 $-$ 2015 reveals that the Seyfert 1
Galaxy, 3C120, has brightened by a magnitude of $1.4$, compared to our campaign
that took place in 2009 $-$ 2010. This allowed us to check for the debated
luminosity and time-dependent color variations claimed for SDSS quasars. For
our 3C120 data, we find that the $B/V$ flux ratio of the variable component in
the bright epoch is indistinguishable from the faint one. We do not find any
color variability on different timescales ranging from about $1$ to $1800$
days. We suggest that the luminosity and time-dependent color variability is an
artifact caused by analyzing the data in magnitudes instead of fluxes. The flux
variation gradients of both epochs yield consistent estimates of the host
galaxy contribution to our 7.5" aperture. These results confirm that the
optical flux variation gradient method works well for Seyfert galaxies.
