Coral reefs are experiencing increased frequency and severity of thermal stress events, underscoring the need for rapid and scalable methods to assess coral bleaching susceptibility. Quantifying bleaching responses typically relies on pulse-amplitude modulated (PAM) fluorometry, an expensive tool that limits widespread implementation. Digital photography, by contrast, is low-cost, accessible, and easily standardized, yet its quantitative potential remains underutilized. Here, we evaluated whether RGB image analysis can serve as an accessible proxy for physiological bleaching metrics using two coral species (Acropora hemprichii and Stylophora pistillata) in an acute heat-stress experiment. RGB values were extracted from coral fragments using a semi-automated segmentation pipeline, and 63 color indices were computed and correlated with PAM-derived Fv/Fm, Symbiodiniaceae cell density, chlorophyll content, and host protein content. We calculated D_toWhite, the Euclidean distance in CIELAB color space between each coral fragment and a co-imaged white reference patch, extracted directly from uncorrected photographs. Across both species, D_toWhite performed nearly identically to the best color-corrected index (DeltaE76). The stronger association of D_toWhite with Symbiodiniaceae cell density and chlorophyll content compared to Fv/Fmreflects their distinct biological axes: D_toWhite tracks Symbiodiniaceae and pigment loss while Fv/Fmreflects photochemical efficiency. Green-weighted indices best approximated Fv/Fm and host protein content, demonstrating that a single standardized image can simultaneously provide information about bleaching severity and Symbiodiniaceae photophysiological state. These results demonstrate that a simple whitening metric derived from uncorrected photographs provides a reliable, scalable, and inexpensive proxy for bleaching severity, supporting higher-throughput thermal tolerance screening in resource-limited settings.