  At present, microlensing light curves from different telescopes and filters
are photometrically aligned by fitting them to a common model. We present a
second method based on photometry of common field stars. If two spectral
responses are similar (or the color of the source is known) then this technique
can resolve important ambiguities that frequently arise when predicting the
future course of the event, and that occasionally persist even when the event
is over. Or if the spectral responses are different, it can be used to derive
the color of the source when that is unknown. We present the essential elements
of this technique and apply it to the case of MOA-2007-BLG-192, an important
planetary event for which the system may be a terrestrial planet orbiting a
brown dwarf or very low mass star. The refined estimate of the source color
that we derive here, V-I=2.36 +- 0.03, will aid in making the estimate of the
lens mass more precise.

Earth and Planetary Astrophysics;Instrumentation and Methods for Astrophysics