In "The Fish" by Elizabeth Bishop, what is meant by "rainbow, rainbow, rainbow"?

It seems
to me this line is representative of
what the narrator of "" figuratively sees.  This
is an important fish, not
just a fish or anyfish. 
This is a
wonderful specimen of age and longevity and perseverence and
character. 

On the outside, though, the fish is not what
anyone would call beautiful-- 

battered
and venerable
and homely. Here and
there
his brown skin
hung in strips
like ancient wallpaper....

It is scarred and battered and lice-ridden and trailing
bits of
other fisherman's lures and lines.  This fish is a testament to age
and strength and character
and perseverence.  While there is an actual
rainbow on the spilled oil around the engine on the
bottom of the boat, the
rainbow in this line is more about the figurative or symbolic nature of
a
rainbow.  This ugly brown trout did not, all of a sudden, regain its coloring; instead,
it is
an image seen by the narrator of what this fish is on the inside.  And
the symbolism of promise
and hope and beauty are all realized in that image
of a rainbow.  Then--"I let the
fish go."