  Freezing of charged colloids on square or triangular two-dimensional periodic
substrates has been recently shown to realize a rich variety of orientational
orders. We propose a theoretical framework to analyze the corresponding
structures. A fundamental ingredient is that a non spherical charged object in
an electrolyte creates a screened electrostatic potential that is anisotropic
at any distance. Our approach is in excellent agreement with the known
experimental and numerical results, and explains in simple terms the reentrant
orientational melting observed in these so called colloidal molecular crystals.
We also investigate the case of a rectangular periodic substrate and predict an
unusual phase transition between orientationnaly ordered states, as the aspect
ratio of the unit cell is changed.
