The self-organization of colloidal nanoparticles (gold, silica, polystyrene…) into intricate and/or periodical structure is a crucial area in colloidal sciences due to their potential to make new functional material. Self-assemblies can be broadly classified into equilibrium and out-ofequilibrium types. Equilibrium assemblies form thermodynamically stable, highly periodic structures that can be used in functional materials like colloidal photonic crystals, metamaterials exhibiting high (n > 3) or negative refractive indices, and optoelectronic metamaterials based on quantum dot superlattices showing super-fluorescence. On the other hand, out-ofequilibrium, or active, self-assemblies respond dynamically to external stimuli (chemical, electrical, optical, magnetic) and offer the advantage of being reconfigurable. In both types of colloidal self-assembly, understanding the interactions between individual components is essential for controlling their spatial arrangement and engineering these materials. Moreover, the relationship between the spatial arrangement, structure and optical properties needs to be unravelled to allow their rational design. Due to their dynamic nature, non-equilibrium assemblies of nanoparticles are inherently more challenging to investigate. Their transient behavior requires real-time assembly formation and simultaneous characterization within a single experimental setup, which increases complexity and limits the range of applicable techniques. Additionally, their dynamics restrict the number of methods available, as even optical microscopy struggles with dense, 3D-packed assemblies, making it difficult to distinguish individual particles. Therefore, new approaches are needed to study the structure, geometry, and properties of non-equilibrium colloidal assemblies.