A general approach to fabricate a magnetic field responsive colloidal crystal film has been developed. This is carried out by depositing monodisperse Fe3O4/PS composite magnetic nanospheres on the surface of an agarose-gel coated substrate. The optical properties of the resultant film can be easily controlled by an external magnetic field, which is caused by assembly of the magnetic nanospheres and alteration of the interparticle distance. With the help of a designed array of small magnets, both the color and pattern of the film can be conveniently modulated and the tuning range covers almost the whole visible spectrum. This work will be important for the potential application of monodisperse magnetic nanospheres in fabricating novel sensors, displays and optoelectronic devices.