  Recently we have witnessed the first multi-messenger detection of colliding
neutron stars through Gravitational Waves (GWs) and Electromagnetic (EM) waves
(GW170817), thanks to the joint efforts of LIGO/Virgo and Space/Ground-based
telescopes. In this paper, we report on the RATIR followup observation
strategies and show the results for the trigger G194575. This trigger is not of
astrophysical interest; however, is of great interests to the robust design of
a followup engine to explore large sky error regions. We discuss the
development of an image-subtraction pipeline for the 6-color, optical/NIR
imaging camera RATIR. Considering a two band ($i$ and $r$) campaign in the Fall
of 2015, we find that the requirement of simultaneous detection in both bands
leads to a factor $\sim$10 reduction in false alarm rate, which can be further
reduced using additional bands. We also show that the performance of our
proposed algorithm is robust to fluctuating observing conditions, maintaining a
low false alarm rate with a modest decrease in system efficiency that can be
overcome utilizing repeat visits. Expanding our pipeline to search for either
optical or NIR detections (3 or more bands), considering separately the optical
$riZ$ and NIR $YJH$ bands, should result in a false alarm rate $\approx 1\%$
and an efficiency $\approx 90\%$. RATIR's simultaneous optical/NIR observations
are expected to yield about one candidate transient in the vast 100
$\mathrm{deg^2}$ LIGO error region for prioritized followup with larger
aperture telescopes.
