Good job beachboy!
I would suggest you post it in the poetry, prose and songs forum.
Regarding the English I can't proofread it because our English levels are practically the same. As far as I see the English is perfect. Depending on the sonority I would suggest to use something like "I wonder where the rainbow could/would begin"
or "I do wonder where the ... " or "I wonder where the (adjective) rainbow begin".
But it is fine the way it is, this is just a suggestion.
Have you already written the music too? Will you post here?