It has been 20 years since the classification of galaxy disk types observed in deep imaging was established. These classifications, which notably consist of pure exponential, truncated, and antitruncated, have significantly enriched our understanding of the structural diversity within galaxies. Using ultra-deep imaging, most spiral galaxy profiles show a clear truncation and a characteristic U-shaped colour profile. However, the origin of this colour profile shape has remained a complete mystery. For spiral galaxies observed edge-on, the minimum point of the U-shape (i.e. the bluest colour) corresponds exactly to the edge (truncation) of the galaxy. This is exactly what we would expect if the edge of the galaxy coincided with the end of star formation. However, for galaxies with lower inclinations, the minimum of the U-shape is surprisingly inside the star-forming disk. How is it possible for both observations to occur simultaneously? In this contribution, we explain different hypotheses and finally, how this puzzle can be solved by critically reviewing the limitations of the current methodology used to obtain structural information in deep imaging surveys. This result sets the theoretical framework for a detailed understanding of the formation of the disk periphery that will be explored by current and future deep imaging surveys.