Color centers in diamond are point defects within the diamond host lattice that absorb and emit light at optical frequencies. Besides contributing to the striking visual characteristics of "fancy colored" diamonds as gemstones, the centers--particularly the negatively charged nitrogen-vacancy (NV) center and silicon-vacancy (SiV-) center--offer a number of possibilities for quantum computation and quantum information processing. In this talk, I will summarize recent progress made in characterizing negatively charged silicon-vacancy centers in diamond using the technique of optical multidimensional coherent spectroscopy (MDCS). By comparing photoluminescence-based and heterodyne-detection based signal collection schemes in a high-density SiV- center sample, we have selectively identified a population of long-lived and nonradiative silicon-vacancy centers in diamond with more than 40 times as much inhomogeneous spectral broadening as the radiative silicon-vacancy center states that are more commonly observed using photoluminescence. Estimates of the degree of inhomogeneity and overall sample characteristics indicate that strain is likely to play a large role in the formation of these nonradiative states. The findings open possibilities for being able to actively tune the degree of radiative coupling in silicon-vacancy center systems, opening possibilities for new types of quantum-optical devices.