Immovable polychrome cultural heritage—including grotto murals, architectural wall paintings, painted plasters, and rock art—presents an analytical problem defined by three simultaneous constraints: the objects are stratified and materially heterogeneous, the key questions are usually conservation-driven rather than purely descriptive, and the investigation must be performed in situ with a level of intervention acceptable to conservation practice. In this context, laser-induced breakdown spectroscopy (LIBS) has moved well beyond its early role as a rapid elemental screening tool. Its practical significance lies in the fact that it can provide elemental information, micrometric sampling, and depth-resolved data within the same analytical sequence, thereby addressing problems that are difficult to solve by fully non-contact techniques alone, which is why depth-resolved investigation is often conservation-critical rather than merely technically attractive. This review focuses specifically on immovable polychrome heritage and examines how LIBS has been used to identify mineral pigments, disentangle repainting and overpaint, diagnose salts and surface deposits, estimate layer thickness, and support decisions on cleaning and consolidation. Emphasis is placed on the analytical logic of field deployment: acceptable micro-destructivity, parameter selection under fragile surface conditions, pulse-to-depth calibration, chemometric interpretation, and the role of multimodal workflows with Raman spectroscopy, X-ray fluorescence, optical coherence tomography, and imaging methods. Recent computational and calibration strategies—numerical simulation, machine-learning-assisted calibration, and micro-CT-aided depth calibration—are also considered as ways to reduce dependence on ideal physical standards when exact mural analogues cannot be prepared. The review argues that the present frontier is no longer whether LIBS can generate useful spectra from wall paintings, but whether its outputs can be standardized, reproduced, and translated into evidence that conservators can trust. Its long-term value therefore depends less on broadening application claims than on consolidating low-damage protocols, uncertainty-aware interpretation, and explicit integration into conservation decision-making.