Anodic aluminum oxide (AAO) thin films were fabricated in phosphoric acid electrolyte solution via one-step electrochemical anodic oxidation of an Al sheet. The films displayed minimally saturated rainbow rings of structural colors, and pores with depths that decreased symmetrically outward from the center of the films. AAO thin films embedded with Co nanodots were fabricated using the one-step ac electrodeposition method. The composite films showed highly saturated rainbow rings of structural colors, which is due to the Co nanodots efficiently screening reflected light from the Al substrate. Moreover, magnetic Co nanodots were found to tune the physical properties of AAO thin film as a new degree of freedom. After determining that Co@AAO composite films possess magnetism and highly saturated rainbow rings of structural colors, their magnetic and optical properties were studied based on modification of the oxidation voltage and deposition time required to prepare the films.