"He held on to the rainbow that was hope" could work, poetically, in some types of writing. However, it doesn't seem a natural or common enough metaphor to be used *****-nilly in ordinary everyday English. If you plonked it into everyday conversation then it would seem strange or pretentious.
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Anonymous
Thanks. Are there other ways of writing it, then? 'He held on to the rainbow called hope' sounds too plain to me.