Among the first settlers was William Porter, who established a school in the late 1860s; for several decades the area was referred to as the Porter school district. By the 1890s a community had developed, and the residents requested a post office. When the townspeople gathered to name the community a thunderstorm broke out, followed by a rainbow. The residents, struck by the beauty of the rainbow, named their town after it. A post office was opened in Rainbow in 1899. By 1910 the community had half a dozen stores, a church, and a school. In the 1920s a newspaper, the Rainbow Reminder, served the estimated 113 residents. Throughout the twentieth century Rainbow has served as a church and school community for area farmers. After the 1920s the population decreased, reaching a low of forty in 1960. By 1981 the number of residents had increased to seventy-six. This growth may be due in part to the opening of nearby Dinosaur Valley State Park in the 1970s. Through 2000 the population was still reported as seventy-six. Source: The Handbook of Texas Online.