I'm trying to get rid of the party clothes I recieved in Diplomatic Immunity, but the clothes are still marked as quest items. Any ideas on how to do it?

Have you completed the quest?

This. you can't remove them until after the party.

I'm all done with the party, but I realize now that I haven't talked to Delphine yet.

Let us know how it goes after that.

*edit* I swore I quoted you. For some reason, I it seems I can't quote anyone for now.

After getting out of the Thalmor Embassy, you have to get the magic zipper from Delphine to get the clothes removed.

Yep, I got rid of them after I talked to her!