  The Wide-field Infrared Survey Explorer (WISE, Wright et al. 2010) and its
follow-up Near-Earth Object (NEO) mission (NEOWISE, Mainzer et al. 2011) scan
the mid-infrared sky twice a year. The spatial and temporal coverage of the
resulting database is of utmost importance for variability studies, in
particular of young stellar objects (YSOs) which have red $W1{-}W2$ colors.
During such an effort, I noticed subarcsecond position offsets between
subsequent visits. The offsets do not appear for targets with small $W1{-}W2$
colors, which points to a chromatic origin in the optics, caused by the
spacecraft pointing alternating ``forward'' and ``backward'' from one visit to
another. It amounts to 0\farcs1 for targets with $W1{-}W2\approx2$.
Consideration of this chromatic offset will improve astrometry. This is of
particular importance for NEOs that are generally red.
