@jerrymich said:
What causes the faces to be reversed?
A common cause when it's things like walls is that the user starts with by drawing the perimeter which creates a face with the back face up. Then they draw lines for the interior walls before extruding them up with P/P. The walls will be inside out and if you orbit to look at the bottom, there's no faces at the bottoms of the walls. Although this might be unexpected, SketchUp is doing the right thing. It's the same as pushing a hole into a face. If you delete the face representing the floor and leave only the face at the bottom of the wall, you'll get the correct extrusion.
Of course you could also reverse the faces at some point. It's a good idea to stay on top of face orientation and fix incorrect orientation as it happens. Much less work and less likely to create problems down the road. Keep in mind the back face color on top of a face drawn on the ground plane is normal since it is expected you'll make it a 3D shape.