<p>Recent years have witnessed growing research interest in two distinctive optical phenomena, photochromism (PhCh) and persistent luminescence (PersL). However, effectively integrating both functionalities into a single host material remains a major challenge, primarily due to the complex role defects play in modulating the material’s optical properties. To address this fundamental issue, this study employs structurally simple CsX (X = Cl, Br) nanocrystals (NCs) as a model system to systematically elucidate the relationship between defects and optical behaviors. We propose for the first time that CsX NCs can accommodate two distinct types of chlorine vacancy defects upon X-ray irradiation—one intrinsic from synthesis and one induced by X-rays—enabling the integration of PhCh and PersL in a single matrix. Specifically, under X-ray irradiation at 20–70 kV, the material exhibits reversible blue coloration, which stems from recoverable chlorine vacancies that can be rapidly eliminated by visible light within 30 s. This PhCh behavior demonstrates excellent cycling stability, with a color difference (ΔRL₁) of 56.9% and a recovery rate (ΔRL₂) of 98.1%. Furthermore, the introduction of Br⁻ effectively deepens the energy level of intrinsic chlorine vacancies from the range of 0.47 to 0.71 eV up to 0.83 eV, resulting in intense PersL that lasts for more than 30 min. Such dual-mode PhCh–PersL characteristics endow these NCs with considerable potential for practical applications, particularly in the field of X-ray colorimetric imaging and dynamic anti-counterfeiting.</p>