Is rainbow a proper noun?

No, the word 'rainbow' is a common noun, a general word for any arch in the sky made up of different colors that forms when the sun's rays are refracted by rain drops; any display or array of different colors.

A proper noun is the name of a specific person, place, or thing.

Examples of proper nouns for the common noun 'rainbow' are:

  • Rainbow Shops, retail apparel chain based in Brooklyn, NY
  • Rainbow Bridge National Monument, Utah
  • "Over the Rainbow", Academy Award winning song from the movie "The Wizard of Oz".

No, rainbow is not considered a proper noun.