A half-mourning dress was a style of dress that was worn by women in the 19th and early 20th centuries during a period of mourning for a loved one. It was typically worn after the initial periods of deep mourning, when the mourner was expected to dress entirely in black.
During the half-mourning period women were allowed to incorporate some muted colors and fabrics other than black into their wardrobe. For example, they might wear gray, mauve, or lavender dresses made of crepe, silk, or other lightweight fabrics. However, the clothing was still relatively subdued and not as elaborate as normal everyday wear.
The half-mourning dress signifed a gradual return to normal life, but with a respectful nod to the deceased. The style evolved with fashion trends of the era, but it generally remained modest and somber in appearance. Here’s some more historical info about the dresses with an example, and also a funny comic inspired by it: