  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.
