Solution conditions conducive to protein crystallization are identified mainly in an empirical manner using screening methods. Measurements of a dilute solution thermodynamic parameter, the osmotic second virial coefficient, have been shown to be useful in guiding this search, yet the measurement of this parameter remains difficult. In this work, a nanoparticle-based assay, self-interaction nanoparticle spectroscopy, is presented as an efficient alternative. The method involves adsorbing proteins on the surface of gold nanoparticles and adding the protein/gold conjugates to solutions of interest for crystallization. The optical properties of gold colloid, including macroscopic ones such as color, are sensitive to the interparticle separation distance, and they are demonstrated to correlate with the value of the second virial coefficient for BSA and ovalbumin. Serendipitously, the conditions that correspond to second virial coefficient values within the thermodynamic region ideal for protein crystallization lead to the maximum change in color of the gold suspensions. Given the remarkable efficiency of this method, it holds significant potential to aid in the crystallization of proteins that have not been crystallized previously. Moreover, this method may find utility in the analysis of weak homo- and heterotypic interactions involved in other biological applications, including preventing protein aggregation and formulating therapeutic proteins.