The issue is: stomachers and petticoats that match each other while contrasting with the gown.
Outlander here serves as a great example, since so many 18th century films are either set in the 1780s or have anachronistic stomacher-less bodices in earlier decades. But others do it too!
The look of a stomacher that matches the petticoat seems to have been really attractive to people through the nineteenth century, and continues to be so up to today! For starters, this illustration dates to only a couple of generations after stomacher'd gowns were being regularly worn:
|Image of marriage from The Stages of Man, ca. 1815; the Abby Aldrich Rockefeller Folk Art Collection|
There are a few period images that show this kind of styling. For instance, Zoffany's 1769 portrait of the Bradshaw family (Tate Collection N06261) shows Elizabeth Wilson Bradshaw in a green gown with a dark pink quilted petticoat and light pink stomacher, sleeve ribbons, and neck frill. Susanna Gale was painted by Reynolds in a costume-looking pink gown with what seems to be a vertically-ruched white petticoat ca. 1763. There are also a few portraits by Francis Cotes, such as this portrait of an unknown lady in 1768:
|Tate Collection N04689|
But apart from a handful of examples, the majority of French, British, and American artwork pre-late 1770s - from portraits to genre prints - shows women of any means matching their petticoats to their gowns (if not also the stomacher, but if there's contrast, that's where it is most of the time). It's a simple detail, but it makes a great deal of difference when someone looks at your reenactment kit or the costume you've designed. Or at least ... it's started to make a great deal of difference to me!
|Mrs. Bowles and her Child, Thomas Hudson, ca. 1755; Cheltenham Art Gallery and Museum 1956.96|
|Mrs. Matthew Michell and her children, Matthew and Anne, Thomas Hudson, 1757-58; LAMS L.F9.1938.0.0|
(an example of the contrasting stomacher effect)
|Mrs. Mary Martyn, Allan Ramsay, 1761; Birmingham Museums Trust 1957P27|
|Sacque, petticoat, and stomacher, ca. 1760; LACMA M.56.6a-b|
|Madame Sophie, Lié Louis Périn-Salbreux, ca. 1773; Rheims Museum of Fine Arts|
|Hannah Choate Lathrop, attr. John Durand, ca. 1775; Historic New England 1991.166.2|