It’s hard to resist the pull of rainbows. Harbingers of sunlight as a storm passes, they’re like ethereal paintings across the sky. In the folklore of countries as distant as Gabon and Japan, as well as in Norse mythology, they bridge Earth and spirit realms. In Ireland, they famously mark the spots of cauldrons overflowing with gold. And in literature and the arts, they frequently symbolize awe or a sense of wonder. They’re marshmallow charms in “magically delicious” children’s cereal, and they’re the namesake of one of the longest enduring counter-cultural groups, established in 1970: The Rainbow Family of Living Light (aka The Rainbow Family). In 1939, Judy Garland as Dorothy sings longingly of a land “Somewhere Over the Rainbow,” where “dreams that you dare to dream really do come true,” and in 1978 the Pride movement first flew the rainbow flag in San Francisco. Romantic poet William Wordsworth wrote in 1802: “My heart leaps up when I behold / A rainbow in the sky: / So was it when my life began; / So is it now I am a man; / So be it when I shall grow old, / Or let me die!” And the rainbow carries special meaning in this area, as well, since the Rainbow Fleet has become an enduring symbol of summertime in Nantucket.

Ironically, the image that so perfectly captures the delightful nature of the Rainbow Fleet, H. Marshall Gardiner’s photo that you see on all of the postcards and prints, was shot on black-and-white film. Helen Wilson Sherman, one of the earliest Rainbow racers, recalled that her uncle, the playwright Austin Strong, helped organize the event. She said, “They...