The rainbow coloured patch is a thin segment of a primary rainbow. 

A small clearing or hole in clouds gives a parallel shaft of sunlight.   The shaft behaves as a single antisolar ray that appears by perspective to converge towards the antisolar point.

The rainbow's centre is also at the antisolar point.   If the shaft of sunlight happens to intersect rain then we get a rainbow segment.

Not all coloured patches are rainbows. Depending on height above the horizon and position of the sun they could be a circumhorizon arc fragment, a small iridescent cloud, a sundog and more..