River wears a pink dress to the funeral at the end of Serenity.
More after the break...
Some more screencaps:
Some pictures of the costume up at the Propstore:
The dress is BCBG and a few people were auctioning similar dresses on Ebay. You can see that one of these below is exactly the same and one of them has a slight variation.
And the slightly altered one:
My Photobucket Album for River.
Serenifly Costumes site for River.