What is the effect of the repetition of "rainbow" in "The Fish" by Elizabeth Bishop?

The next-to-last line, a trio of the"rainbow," actually begins a few lines
earlier. The final section of this poem reads,

I stared
and stared
and victory filled up
the little rented boat,
from the
pool of bilge
where oil had spread a
rainbow
around the rusted engine
to the bailer
rusted orange,
the sun-cracked thwarts,
the oarlocks on their
strings,
the gunnelsuntil everything
was rainbow, rainbow,
rainbow!
And I let the fish go.

First, the rainbow...