Variation of the stellar color in high-magnification and
  caustic-crossing microlensing events
  To a first approximation, the microlensing phenomenon is achromatic, and
great advancement has been achieved in the interpretation of the achromatic
signals, which among other achievements has led to the discovery and
characterization of well above $100$ new exoplanets. At higher order accuracy
in the observations, microlensing has a chromatic component (a color term)
which has so far been much less explored. Here, we analyze the chromatic
microlensing effect of $4$ different physical phenomena, which have the
potential to add important new knowledge about the stellar properties not
easily reachable with other methods of observations. Our simulation is limited
to the case of main-sequence source stars. Microlensing is particularly
sensitive to giant and sub-giant stars near the Galactic center. While this
population can be studied in short snapshots by use of the largest telescopes
in the world, a general monitoring and characterization of the population can
be achieved by use of more accessible medium-sized telescopes with specialized
equipments through dual-color monitoring from observatories at sites with
excellent seeing. We quantify our results to what will be achievable from the
Danish $1.54$m telescope at La Silla observatory by use of the existing
dual-color lucky imaging camera. Such potential monitoring programs of the
bulge population from medium-sized telescopes include the characterization of
starspots, limb-darkening, the frequency of close-in giant planet companions,
and gravity darkening for blended source stars. We conclude our simulations
with quantifying the likelihood of detecting these different phenomena per
object where they are present to be $\sim 60\%$ and $\sim 30\%$ for the
mentioned phenomena, when monitored during high magnification and caustic
crossings, respectively.
