Abstract Understanding transport of anomalous colloids and contaminants requires partition of polydisperse colloids at the pore scale. Using a high‐resolution dual‐mode microscopy system, we directly visualize cross‐sectional concentration profiles. Furthermore, we identify a previously overlooked phenomenon: the occurrence of persistent transverse concentration heterogeneity and centerline enrichment above the injected concentration (C/C0 > 1) during transport. Observations at different spatial and temporal scales reveal transverse colloid migration during anomalous segregation and enrichment of colloids. These observations revealed that colloid transport results in the formation of distinct concentration‐defined regions that cannot be inferred from conventional solute transport behavior. We establish a double‐erf logarithmic‐gradient criterion to delineate the main‐flow and diffusion zones and quantify the main‐flow ratio (β $beta $). This criterion defines transport zones directly from the observed concentration gradients, avoiding the nonuniqueness of conventional inversion‐based two‐zone models. Our results provide direct pore‐scale evidence and a quantitative framework linking pore‐scale transport structure to macroscopic behavior.