This effect was generated in Filter Forge, a Photoshop plugin that allows you to create your own filters.
This filter can generate a variety of rainbows. I pictured it being useful to use with landscape shots where you want to add some extra excitement like having the end of a rainbow go to the focal point of the composition or use in shots where a rainbow would help convey more about the atmospheric condition. Ofcourse you could just put it like a halo around someone's head just for fun too. The rainbow can be made thicker or thinner, moved farther away and moved around the canvas so you can put it where you want. The filter has settings to allow you to make the effect pretty subtle as well.
rainbow landscape photo photography clouds water color transparent special happy leprechaun gold rain storm inspiration