The black γ‐phase of CsPbI3 with its bandgap at ≈1.75 eV can enable the take‐off of tandem solar cells as soon as intrinsic instability to the yellow δ‐phase is solved. Here, a black γ‐phase is formed at low temperature (80–90 °C) by incorporating Eu through EuCl3 or EuI2. It is forced to become yellow by thermal heating under nitrogen. It is demonstrated how spectroscopic ellipsometry and the related critical points analysis provide a new diagnostic tool to monitor the transformation through the bandgap footprint. As two sides of the same coin, the consumption of the black γ‐phase and the growth of the yellow δ‐phase are addressed and modeled in the range of 60–100 °C using the Avrami's theory. This approach allows extracting the activation energies of the phase transformation that are 0.95 eV versus 1.15 eV using EuI2 and EuCl3, respectively. In situ transmission electron microscopy analyses …