(It's always wonderful to find a well-documented photograph of someone in a fancy dress outfit. Here's one from the era of the hoop skirt; how appropriate it is to find a photograph of Photography! Click the image for a larger version.)
Left: Miss Stevenson, as "Photography", Montréal, QC, 1865 by William Notman. Image © McCord Museum.
The photograph is part of an online exhibition of photographs of fancy dress costumes from balls and skating parties; the museum's description of it states:
"Miss Stevenson attended a fancy dress ball in Montreal in 1865 at the Theatre Royal, given by the officers of the garrison. She dressed as "Photography", complete with a camera on her head and photographs adorning her dress, shoes, bracelets and fan. Miss Stevenson copied this dress from a printed fashion plate, which frequently offered images of fancy dress costumes. The almost identical costume in the Parisian plate is bright green."
While I do not, alas, have the fashion plate referred to, I do have two nearly-identical descriptions of a photography costume, with details very similar to those shown in the photograph, from the second and fifth editions of a fancy dress manual from the 1880s. They confirm the suggestion above that the dress would be green. Presumably were the costume to be made in the style of the 1880s the short-for-its-era skirt would take the shape of a bustle rather than a hoop. The "lunette" referred to in the descriptions is a half-moon shape rather than the camera used by Miss Stevenson.
(1) A green silk dress trimmed with tulle of the same shade, round the skirt, nestling in the bouillonnés, a row of photographs, a scarf of the silk draped across the skirt, with medallion photographs at intervals, all bordered with green gimp, the bertha of the low bodice fastened at the front, back, and on the shoulders with them. A cap in the form of a lunette, with cartes de visites, and a long green veil depending.
(2) A green silk dress trimmed with tulle of the same shade; round the skirt, nestling in the bouillonnés a row of photographs; a scarf of the silk draped across the skirt, with medallion photographs at intervals, all bordered with green galon; the bertha of the low bodice fastened at the front, back, and on the shoulders with them; a cap in the form of a lunette, with cartes-de-visite, and a long green veil depending.
Sources:
(1) Holt, Ardern. Fancy Dresses Described, 2nd Edition, Illustrated. London: Debenham & Freebody, 1880.
(2) Holt, Ardern. Fancy Dresses Described, 5th Edition. London: Debenham & Freebody, 1887.