  [Abridged] The giant molecular clouds (GMCs) of external galaxies can be
mapped with sub-arcsecond resolution using multiband observations in the
near-infrared. However, the interpretation of the observed features, and their
transformation into physical quantities, is greatly hampered by the effects
arising from the unknown geometry and the scattering of light by dust
particles. We examine the relation between the observed NIR reddening and the
column density of the dust clouds. In this paper we particularly assess the
feasibility of deriving the mass function of GMCs from NIR color-excess data.
We perform Monte Carlo radiative transfer simulations with 3D models of stellar
radiation and clumpy dust distributions. We include the scattered light in the
models and calculate NIR color maps from the simulated data. We extract clumps
from the color maps and compare the observed mass function to the true mass
function. For the physical configuration chosen in this study, essentially a
face-on geometry, the observed mass function is a non-trivial function of the
true mass function. The dynamical range of the observed mass function is
confined to ~10^3.5... 10^5.5 M_\odot regardless of the dynamical range of the
true mass function. The color maps are more sensitive in detecting the
high-mass end of the mass function, and on average the masses of clouds are
underestimated by a factor of ~10. The simulations show that the cloud mass
function derived from JHK color excess data using simple foreground screening
geometry cannot be regarded as a one-to-one tracer of the underlying mass
function.
