promise vs rainbow

promise and rainbow both are nouns.

Word	Noun	Adjective	Verb	Adverb
promise	Yes	No	Yes	No
rainbow	Yes	No	No	No

As nouns, rainbow is a hyponym of promise; that is, rainbow is a word with a more specific, narrower meaning than promise:

promise (noun)	rainbow (noun)
grounds for feeling hopeful about the future	an illusory hope
a verbal commitment by one person to another agreeing to do (or not to do) something in the future	an arc of colored light in the sky caused by refraction of the sun's rays by rain

promise (verb)	rainbow (verb)
make a promise or commitment
promise to undertake or give
make a prediction about; tell in advance
give grounds for expectations