It's probably impossible for me to find a single fashion plate that mirrors Miss Bingley's saffron ball gown, but I did find this 1805 plate of full dress from The Lady's Magazine. It has many of the same characteristics: an open-fronted robe over a petticoat, short sleeves with fanciful oversleeves, and the same sort of twisted up/negligent curls hairstyle decorated with a scarf (Miss Bingley adds feathers).

Although the book on making the film quotes the designer as saying she was looking for 1813 styles, the whole "feel" is much earlier, more like 1805. However, if Miss Bingley's dress were truly to be an 1805 dress, the petticoat should be white, either satin or muslin, not coloured.

Louisa
Mistress of the Garderobe