We present a technique using ferrofluid to induce bidisperse suspensions of superparamagnetic and diamagnetic beads to assemble into colloidal ring configurations. The separation distance between particles within the ring can be tuned by adjusting the ferrofluid concentration, which has the effect of enhancing the effective dipole moment of one of the components while screening the dipole moment of the other, leading to a wealth of different ring configurations. © 2010 American Institute of Physics.