To establish trust in an optical measurement as a proxy for phase change, the limits within which optical proxies are valid are analyzed and possible false relationships identified. As the authors did not have access to an in-situ XRD tool, we established the following correlations: (1) For ensembles of samples, we compared centroids of pre-degradation XRD data with the centroids of the time-resolved RGB-camera-based measurements, using clustering methods to identify centroids (Figure ). ( ) For individual samples, we measured XRD before and after degradation, and compared to color changes (Figure ). We also reference a prior study that examines changes in perovskite RGB color change with solar-cell device efficiency, shown in Supplemental Information Figure . The limits of this color-based proxy are explored in the last paragraph of this sub-section.