We have conducted a study on the nonlinear absorption properties of nitrogen-vacancy color centers in processed nanodiamonds. Their two-photon (2P) spectra disclose distinguishable features for the two charge states in which the center exists. The 2P absorption cross section is found to be between 0.1 and 0.5 GM in the wavelength range between 800 and 1040 nm. In addition, the center demonstrates the feature of strong 2P absorption for its neutral charge state below 1000 nm excitation wavelength and predominant 2P absorption by the negative charge state above this wavelength.