That usually means the surfaces' normals are opposite.

EDIT:

Woot! 5000 posts!

Just so I can keep 5000 for a little while longer, I'll answer your question up here. lol

With the model selected, go to Display > Polygon Component Display > and choose whichever kind of normal display you'd like. Choose it again to turn it off.

Since the model is one object, you'll need to select the faces of whichever part and Edit Polygons > Normals > Reverse.

You generally want the normals facing out.