Like most gals my age, my first introduction to a stomacher was in the book "Felicity's Surprise," the third book in the Felicity series from the American Girl Collection. Felicity was always my favorite of the (original) American Girls because her style was so awesome. If I remember correctly, Felicity gets a really fancy dress made at Christmas complete with a stomacher. In the American Girl catalog, the stomacher was reversible- one side had white details and the other had pink. I wanted Felicity's Christmas outfit more than anything and since this is real life and not a novel, I never got it. Felicity also wore a contrasting red cloak with her gown on the way to the ball which is also historically correct. Those American Girl people really do their research.
Here is a picture of two actual stomachers. These are a bit blander than my American Girl's but hey, it wasn't balls all the time back then.