The Huggins coefficient, which characterizes the exact, dilute limiting viscosity of colloidal suspensions with square-well interactions, is calculated. An overview of the method and numerical results are presented for both monodisperse and bidisperse suspensions of spherical colloids. The results demonstrate that attractions increase the viscosity much more than repulsions of similar magnitude, Further, the viscosity of polydisperse suspensions of these particles is very sensitive to the smaller size fraction. These calculations complement other theoretical work on the equilibrium thermodynamic properties and colloidal diffusion to provide a more complete reference state for analyzing the role of colloidal potentials on suspension properties.