Georgette Heyer: The Masqueraders

The Masqueraders, by Georgette Heyer, is a Georgian romance and adventure about two siblings, Prudence and Robin Lacey. They masquerade as the opposite sex in order to foil political enemies, but the fun begins when they fall in love with others under this masquerade.

Allan Kass shows us one of the "couples" with our heroine dressed as a man and presumably the hero is the man with whom she has fallen in love. Note the bright yellow and red colors which wouldn't have been used in men's dress during the regency, unless it was part of a uniform.