This project explores how depletion interactions—induced by adding micelles or polymers—can be used to manipulate the stability and aggregation of colloidal particles. By tuning surface chemistry and depletion strength, we guide the transition from stable suspensions to selective aggregation and precipitation. These controlled interactions underpin techniques like Bibette sorting for colloidal fractionation and depletion-induced structural coloration, where color arises from particle assembly rather than pigments. This work connects soft matter physics with practical applications in materials processing, optics, and formulation science.