We simulate the complete 2MASS Ks vs. J-Ks data towards the LMC by means of a population synthesis approach based on extended libraries of published stellar evolutionary tracks. The simulations provide a quite detailed description of the several features (vertical ``fingers'' and inclined sequences) seen in 2MASS data, due both to the Galactic foreground and to LMC O-rich stars. The only exception is the complete absence of the red tail of C-stars in such models. We then demonstrate that the discrepancy is simply removed when we adopt new evolutionary models that include the changes in molecular opacities as the stars get enriched in carbon along the AGB. In fact, simulations which adopt these variable opacities are able to reproduce, for the first time, the red plume of C-stars in near-infrared CMDs.