Khaki. Why? Easier. Also, the khaki uniform is better than the red coat and blue trousers. Why? Cuz if you need them to be FFL, or Belgians, Germans, whatever, they'll do in a pinch; just switch out the flags. Redcoats are pretty, but they can only be British.
Incidentally, the largest collection of Colonial minis I have are blue and white jacketed naval infantry. These uniforms are almost universal. You can even use them as Americans, or as shore parties from merchant ships that run aground off the shores of mysterious islands filled with dinosaurs, undead, or mad scientists building armies of beastmen or clockwork soldiers.
All hail versatility!