The lining in my orange Eiffel Tower bags are white. I got khaki last year - to me the color looks the same as what is out now, though I have not done a careful comparison.
I was so intent on getting the orange that I had a friend (whose business is shopping in France) pick up 2 styles months ago and I got 2 more when I was in Paris in Jan.
I can't speak to which is the best LC store in Paris. I go to the one on FSH which is new-ish and large. I also pop into a smaller store on the Left Bank, though I did not visit this particular branch in Jan. There are also Longchamp boutiques in dept stores-I have been to the one in Galleries Lafayette.
As far as duty free shopping, you need to do that as you leave, not on arrival. You have to show boarding passes for flights leaving the EU. I am not sure what happens if you are leaving France, but are going elsewhere in the EU.