The phrase "as a rainbow" is correct and usable in written English.
It can be used in contexts where you want to describe something that is colorful, vibrant, or diverse, often metaphorically.
Example: "Her personality shone through as a rainbow, bringing joy and brightness to everyone around her."
Alternatives: "like a spectrum" or "as colorful as a rainbow."
Even with a slightly ragged corps de ballet, "Divertimento" is as beautiful as a rainbow.
I like to think of our identity as a rainbow; not tangible, nor static or whole.
But they were impressed by the powerful drawing options, which include such wonders as a rainbow crayon.
I sit on the damp wooden steps of my hut and watch as a rainbow lights up the sky.
A rare-earth sensor embedded in a YSZ coating prepared by electron-beam deposition is demonstrated as well as a "rainbow" sensor.
Mr Obama is much more exciting than Mrs Clinton: a charismatic black man who embodies the Democratic Party's self-image as a rainbow party.
It isn't cheap -- almost nothing here is, by Mexican standards -- but the shrimp, prepared a dozen different ways, are stellar, as are a rainbow of homemade salsas.
Don't choose a rainbow, as Rainbow Dash has already taken that.
Still, the film is buoyant, popping with a hopeful California brightness: the Japanese version's title translates as "A Rainbow-Colored World for You and Me".
The music playfully references 8-bit videogame soundtracks, as well as folk, R&B and hip-hop, while the likes of Emo Philips, Kristen Schaal and Henry Rollins – as a rainbow-coloured unicorn, no less – provide the voices.
The musical, after all, offers a decidedly patriotic American perspective on the Revolutionary War, with the Founding Fathers presented as a rainbow-hued band of immigrant brothers rapping against King George III, who is depicted as a foppish British Invasion crooner.
