In summary, color change is established as a reliable proxy for degradation in perovskite bulk samples, thin films and solar cell devices, and has aided researchers to improve perovskite device stability. This proxy is valid within the following limits: (1) the decomposition phases are optically distinguishable from the original film at the wavelengths selected for the hardware (light source and imaging system), the lag between color change and device efficiency decrease is sufficiently small, and (3) bulk samples have a sufficiently large surface area to volume ratio such that surface degradation is representative of overall degradation.