  Meeting the science goals for many current and future ground-based optical
large-area sky surveys requires that the calibrated broadband photometry is
stable in time and uniform over the sky to 1% precision or better. Past surveys
have achieved photometric precision of 1-2% by calibrating the survey's stellar
photometry with repeated measurements of a large number of stars observed in
multiple epochs. The calibration techniques employed by these surveys only
consider the relative frame-by-frame photometric zeropoint offset and the focal
plane position-dependent illumination corrections, which are independent of the
source color. However, variations in the wavelength dependence of the
atmospheric transmission and the instrumental throughput induce source
color-dependent systematic errors. These systematic errors must also be
considered to achieve the most precise photometric measurements. In this paper,
we examine such systematic chromatic errors using photometry from the Dark
Energy Survey (DES) as an example. We define a natural magnitude system for DES
and calculate the systematic errors on stellar magnitudes, when the atmospheric
transmission and instrumental throughput deviate from the natural system. We
conclude that the systematic chromatic errors caused by the change of airmass
in each exposure, the change of the precipitable water vapor and aerosol in the
atmosphere over time, and the non-uniformity of instrumental throughput over
the focal plane, can be up to 2% in some bandpasses. We compare the calculated
systematic chromatic errors with the observed DES data. For the test sample
data, we correct these errors using measurements of the atmospheric
transmission and instrumental throughput. The residual after correction is less
than 0.3%. We also find that the errors for non-stellar objects are
redshift-dependent and can be larger than those for stars at certain redshifts.
