  [Abridged] We describe the near-infrared reddening signature of giant
molecular clouds (GMCs) in external galaxies. In particular, we examine the
E(J-H) and E(H-K) color-excesses, and the effective extinction law observed in
discrete GMC regions. We also study the effect of the relative scale height of
the GMC distribution to the color-excesses, and to the observed mass function
of GMCs. We perform Monte Carlo radiative transfer simulations with 3D models
of stellar radiation and clumpy dust distributions, resembling a face-on
geometry. The scattered light is included in the models, and near-infrared
color maps are calculated from the simulated data. The effective near-infrared
reddening law, i.e. the ratio E(J-H)/E(H-K), has a value close to unity in GMC
regions. The ratio depends on the relative scale height of GMCs, xi, and for xi
values 0.1...0.75 we find the typical ratios of 0.6...1.1. The effective
extinction law turns out to be very flat in GMC regions. We find the ratios of
apparent extinctions of A(H)/A(K)=1.35...1.55 and A(J)/A(H)=1.15. The effect of
the scattered flux on the effective reddening law, as well as on the effective
extinction law, is significant. Regarding the GMC mass function, we find no
correlation between the input and observed slopes of the mass functions.
Rather, the observed slope reflects the parameter $\xi$ and the dynamical range
of the mass function. We estimate that only a fraction of 10...20 % of the
total mass of GMCs is recovered, if the observed color-excess values are
transformed to masses using the Galactic reddening law. In the case of
individual clouds the fraction can vary between ~0...50 %.
