  We analyze very deep HST, VLT and Spitzer photometry of galaxies at 2<z<3.5
in the Hubble Deep Field South. The sample is selected from the deepest public
K-band imaging currently available. We show that the rest-frame U-V vs V-J
color-color diagram is a powerful diagnostic of the stellar populations of
distant galaxies. Galaxies with red rest-frame U-V colors are generally red in
rest-frame V-J as well. However, at a given U-V color a range in V-J colors
exists, and we show that this allows us to distinguish young, dusty galaxies
from old, passively evolving galaxies. We quantify the effects of IRAC
photometry on estimates of masses, ages, and the dust content of z>2 galaxies.
The estimated distributions of these properties do not change significantly
when adding IRAC data to the UBVIJHK photometry. However, for individual
galaxies the addition of IRAC can improve the constraints on the stellar
populations, especially for red galaxies: uncertainties in stellar mass
decrease by a factor of 2.7 for red (U-V > 1) galaxies, but only by a factor of
1.3 for blue (U-V < 1) galaxies. We find a similar color-dependence of the
improvement for estimates of age and dust extinction. In addition, the
improvement from adding IRAC depends on the availability of full near-infrared
JHK coverage; if only K-band were available, the mass uncertainties of blue
galaxies would decrease by a more substantial factor 1.9. Finally, we find that
a trend of galaxy color with stellar mass is already present at z>2. The most
massive galaxies at high redshift have red rest-frame U-V colors compared to
lower mass galaxies even when allowing for complex star formation histories.
