  Astrometry has long been a promising technique for exoplanet detection. At
the theoretical limits, astrometry would allow for the detection of smaller
planets than previously seen by current exoplanet search methods, but stellar
activity may make these theoretical limits unreachable. Astrometric jitter of a
Sun-like star due to magnetic activity in its photosphere induces apparent
variability in the photocenter of order $0.5\ \textrm{m}R_\odot$. This jitter
creates a fundamental astrophysical noise floor preventing detection of
lower-mass planets in a single spectral band. By injecting planet orbits into
simulated solar data at five different passbands, we investigate mitigation of
this fundamental astrometric noise using correlations across passbands. For a
true solar analog and a planet at 1 au semimajor axis, the $6\sigma$ detection
limit set by stellar activity for an ideal telescope at the best single
passband is $0.01$ Earth masses. We found that pairs of passbands with highly
correlated astrometric jitter due to stellar activity, but with less motion in
the redder band, enable higher-precision measurements of the common signal from
the planet. Using this method improves detectable planet masses at 1 au by up
to a factor of $10$, corresponding to at best $0.005$ Earth masses for a
Sun-like star with a perfect telescope. Given these results, we recommend that
future astrometry missions consider proceeding with two or more passbands to
reduce noise due to stellar activity.
