For example, I wanted the sequences set decades ago to be visually distinct from the rest of the book, and so gave them a completely different palette. Because the main sections of the book use autumn colors, it seemed natural to go for a spring palette for the past, hence all the greens. Then I also needed the underwater confrontation to have a different feel, indicating that Mirka had traveled into the Fish's world, and so that ended up being all in blues. And, finally, I wanted the Fish to be a visual alien wherever she was, and to "pop" visually, and after quite a bit of trial and error she wound up being orange-colored in the green past and green-colored in the orange present.
When I first started writing this story, the villain was originally a magic chicken. But then I was inspired to use a magical fish character by a 2003 news story in New York, in which some Hasidic Jews reported hearing a carp in a fish market yell in Hebrew. This eventually got mixed up with the old fairy tale "The Fisherman and His Wife," about a wish-granting Fish - the Brothers Grimm collected that fairy tale, among others. Plus at some point my mind latched onto an image of a little girl whose head had been replaced by a giant fish body. So, as usual, I sort of started with this mix of elements and just kept on shaking them together until something story-shaped came out.