Correction to the photometric colors of the Gaia Data Release 2 with the
  stellar color regression method
  The second Gaia data release (DR2) delivers accurate and homogeneous
photometry data of the whole sky to an exquisite quality, reaching down to the
unprecedented milli-magnitude (mmag) level for the G, GRP, and GBP passbands.
However, the presence of magnitude-dependent systematic effects at the 10 mmag
level limits its power in scientific exploitation. In this work, using about
half-million stars in common with the LAMOST DR5, we apply the
spectroscopy-based stellar color regression method to calibrate the Gaia G-GRP
and GBP-GRP colors. With an unprecedented precision of about 1 mmag, systematic
trends with G magnitude are revealed for both colors in great detail,
reflecting changes in instrument configurations. Color dependent trends are
found for the GBP-GRP color and for stars brighter than G~11.5 mag. The
calibration is up to 20 mmag in general and varies a few mmag/mag. A revised
color-color diagram of Gaia DR2 is given, and some applications are briefly
discussed.
