Context :

Aims : We describe the near-infrared reddening signature of giant molecular clouds ( GMCs ) in external galaxies .
In particular , we examine the E _ { \mathrm { J - H } } and E _ { \mathrm { 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 when the masses are derived using color-excess as a linear estimator of mass .

Methods : 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 .
We perform the simulations with different scale heights of GMCs and compare the color-excesses and attenuation of light in different geometries .
We extract GMCs from the simulated color maps and compare the mass functions to the input mass functions .

Results : The effective near-infrared reddening law , i.e .
the ratio E _ { \mathrm { J - H } } / E _ { \mathrm { H - K } } , has a value close to unity in GMC regions .
The ratio depends significantly on the relative scale height of GMCs , \xi , and for \xi values 0.1 \dots 0.75 we find the typical ratios of 0.6 \dots 1.1 .
The effective extinction law turns out to be very flat in GMC regions .
We find the ratios of apparent ectinctions of A _ { \mathrm { H } } ^ { \mathrm { a } } / A _ { \mathrm { K } } ^ { \mathrm { a } } = 1.35 \dots 1.55 and A _ { \mathrm { J } } ^ { \mathrm { a } } / A _ { \mathrm { H } } ^ { \mathrm { a } } = 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 .
As the observed slope depends on the geometric parameters which are not known , it is not possible to constrain the slope of the mass function using this technique .
We estimate that only a fraction of 10 \dots 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 \sim 0 \dots 50 \% .

Conclusions :