A new expression is given for the long time diffusion coefficient D_L(k) of charged interacting colloidal spheres in suspension, as a function of the wavenumber k, near k=k_m, where the static structure factor has a maximum. The expression is based on a physical analogy between a mode description of the behaviour of atomic fluids (as observed in neutron scattering) and of colloids (as observed in light scattering). Use of this expression in conjunction with a hard-sphere model yields good agreement with extant data on colloids.