Typically, Elizabeth Bishop's poetry is pictorial, not only in the sense of giving vivid descriptions of natural phenomen, but also in the poem's use of such objects that reflect on the self referential. Thus, the rainbow represents victory for both the fish and its capturer. In her delicacy of language, however, Bishop does not overtly communicate what this victory encompasses. Instead, in her poem "The Fish," she employs ambiguity "rainbow, rainbow, rainbow." Beauty comes from destruction.
Yet, in the victory of both the fish and its capturer, the exclamation becomes both abstract and concrete. As in a painting, the rainbow is concretely visible on the fish, while it is felt by the capturer who has caught it, relishing her victory over catching such a venerable fish who has escaped other fishermen as well as the fish's ability to have eluded capture so many times. The emotions are metaphorically a rainbow as the capturer experiences both bravado and sympathy:
I caught a tremendous fish... (bravado)
I admired his sullen face...Like medals with their ribbons
frayed and wavering,(sympathy)....
I stared and stared
and victory filled up
the little rented boat.
"The Fish" is a poem overflowing with imagery. This particular line, of course, comes right at the end of the poem, right before the fish is released. I tend to think the line "everything was rainbow, rainbow, rainbow!" has to be abstract.
To read it as a concrete image is to imply that all the narrator saw was rainbow on the fish or in the sky (for which there is no evidence). We know there is an actual reflected rainbow in the oil sitting in the bottom of the boat a few lines before this one; it seems unlikely the line refers either to that or the suddenly overpowering striping of the rainbow trout (which is, in reality, less a rainbow than just a red strip of color).
To read it as a figurative image implies all the symbolism and meaning traditionally attributed to a rainbow: beauty, hope, dreams, promise. These seem much more likely to be the view of a narrator who appreciates the character and perseverence of a fish that his battled its way through life and the intention of release than the more literal rainbow stripes.
The title tells us this is not just any fish ("The Fish"), and the opening lines tell us:
I caught a tremendous fish
and held him beside the boat
half out of water, with my hook
fast in a corner of his mouth.
Our narrator was not initially struck with its colorfulness; in fact, the description is of "brown skin hung in strips like ancient wallpaper." There is no literal rainbow to be seen on this battered and scarred yet beautiful fish--
--until everything was
rainbow, rainbow, rainbow!
And I let the fish go.
The rainbow seems so symbolic of the respect our narrator has for this creature (which shows itself in this new perspective on a battered, ugly fish) as well as the hope for the fish's future.