Then in that case, it would work better universally to implement this in the UI.

As to how... does the DirectX draw allow for the rendering of polys with separately colored sides?

I think I remember seeing in Rhino, that they achieved the same effect by showing little hash lines emanating from the middle and normal face with a little arrow.
That can be drawn in and out as the temporary construction lines behave.

A check switch in the option's view section could suffice to activate this demarcation feature.
Therefore, we can leave it to the designer to choose which surfaces to "correct" by selecting and Flipping.