Easiest way is to create a “map” of the rainbow coloring you want to add and put that in a new layer on top, mask out the areas you don’t want to have rainbow, likely set the blend to “color” and then vary the opacity.
You can make a bubbly rainbow pattern with a huge variety of tools – render clouds, add a rainbow gradient map, use ripple, glass or some other distortion, or swirl or what have you, etc.
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Jonathan Ziegler
http://www.electrictiger.com
520-360-8293