Arwen's flowing, Greek-chiton-style lingerie in this scene is beautiful - I wouldn't mind it for myself !

In some shots it is a periwinkle-blue, in others, a soft purple color.  Let's go with the consensus view of other websites and say that it's purple.

It appears to be held on by one point on each shoulder, draping loosely in front.  The fabric is very sheer, almost see-through.  The dress is seamless, with the straps of the dress fitting tightly to support the breast area.

The dress is in two layers, which is the part where a person who failed Home Economics like me gets confused.  For more details on this dress for all you very clever dressmakers out there (and even details on how to make one for yourself), please visit alleycatscratch's page for details.

In this picture, one is able to make out a sort of beading pattern on the straps near the shoulders.

This picture gives a lot of detail for the below-the-waist
detail of the dress.

<--- Good shot of the front of the dress.

Here you can see 
the beading on the neckline  of the dress. ---> 

<--- A shot of the back of the dress.

Much thanks to The Costumer's Guide for the pics !