In addition to changing the radius and lattice constant of colloidal photonic crystals to achieve the modulation of photonic bandgap, it is also an important scientific problem that whether the chromogenic behavior can be precisely controlled by the mixing of various size particles. By means of numerical simulation, on the basis of analyzing the dependence of Fe3O4@SiO2 colloidal photonic crystal bandgap range on dielectric constant, electromagnetic wave incidence angle, lattice constant, particle size, and SiO2 coating thickness, the optical transmission characteristics of photonic crystals obtained by mixing particles of varied sizes were studied. The simulation results found that after the magnetic colloidal particles of two particle sizes are mixed at different mass ratios, the photonic bandgap position always falls between the bandgap positions of the photonic crystals formed by the two colloidal particles with different sizes. The reflection spectra gradually shift to red as the doping ratio of the large particle size increases. This result proves that the principle of color mixing is still applicable to colloidal photonic crystals. It has an important reference value for the study of new adjustment methods of colloidal photonic crystal structural color.