Lucilla's Party Costume
These images were brightened to show more detail:
We also see this dress under her cloak in the carriage:
Long-sleeved green v-neck dress with embroidery or trim around the neck and the cuffs of the sleeves. A little bit of blue fringe sticks out from the cuffs. Either an armband or more embroidery appears above her elbow. There is something indistinct below her chest, possibly a mock armor cuirass as in the concept art. It also exists with her Blue Germania costume. A purplish wrap is pinned to her shoulder.
I've been trying to track down what Lucilla wears over the dress on her Party Frock and her Blue Germania costume. I finally have some screencaps of what I think I'm seeing. It seems to be a gold or brown corset, though I don't have any concrete details. I think I'm seeing a corset with gold embroidery and a center front opening. I suspect that it's meant to be some sort of armor for her, as she wears it the whole time she's in Germania and then we never see it again. You can see it best in this images: