It is a rare occasion when we can compare a gown in a portrait with the actual dress. The painting, after George Daw, of the Prince Regent’s daughter, Princess Charlotte, shows her wearing a charming blue gown in a domestic, albeit royal, setting.
A second portrait depicts the same dress more royally. Princess Charlotte (or the artist) has accessorized the dress with ermine, lace, and pearls.

Image @Grand Ladies
The mannequin from the Museum of London exhibit a few years back is dressed more informally, as if she was in the morning room reading. I found the image on Pinterest, but unfortunately the pin did not state the image’s origin. (A reader wrote that the dress belongs to the Royal Collection.)
As you can see, the dress no longer possesses the rich blue hue as shown in the paintings. So many of the costumes of that era are not only in a fragile state and can rarely be shown, but we cannot trust that the colors have remained the same.

Princess Charlotte's "Russian" outfit as shown at The Museum of London. The gown belongs to The Royal Collection.
Below is the original portrait by George Daw, which shows Princess Charlotte wearing the same dress. Click here for yet another view of Charlotte in a similar gown, but without the trim and wearing a lace cap. My sense is that after her death Princess Charlotte’s image became sought after and that many portrait copies were made (both in oil and in print) to satisfy the mourning public.

Princess Charlotte, George Daw, 1817. Image @National Portrait Gallery
Find more views of the gown at Jenny La Fleur’s site. Images of the gown can be seen in the exhibit catalogue called In Royal Fashion: Clothes of Princess Charlotte of Wales and Queen Victoria, 1796-1901, which can only be obtained second hand.
The exhibit: Princess Charlotte, The Lost Princess, will be on display at the Prince Regent Gallery in the Brighton Pavillion through 10 March, 2013.
My other posts about Princess Charlotte: