I’ve never found a better website for information about rainbows and other optical phenomena than Atmospheric Optics. They have information about all flavors of rainbows including the primary rainbow, and explain that rainbows are disks of light rather than sets of coloured rings:
Early morning and late afternoon are the best times to see them because the sun must not be too high. Rainbows are always opposite the sun and their centres are below the horizon at the the antisolar point. The lower the sun the higher is the bow.
Red is always outermost in the primary bow with orange, yellow, green and blue within. Occasionally, when the raindrops are small, fainter supernumerary arcs of electric greens, pinks and purples lie just inside the main bow.
About this particular rainbow, Jamie writes:
This is the first time I have ever seen part of a rainbow in open skies. Look to the sky above the barn and you can just make out the missing portion of the rainbow.
More rainbows on Michigan in Pictures!