I note that rainbows are essentially iridescent clouds, in which the clouds aren't visible, due to water droplets in the air which refracts light. Clouds are composed of water droplets. Unless you note some rippling of the colors in the iridescent clouds, what you are describing as iridescent clouds are basically partial rainbows, where most of the air is devoid of water droplets and only the cloud is refracting the light due to water being present in the cloud, IMHO.

Thanks!