(as|) red as a beet

turning red in the face out of embarrassment

The origin of the idiom "red as a beet" is not definitively known, but it likely comes from the observation of the physical reaction of a person's face turning red when experiencing embarrassment or shame. The comparison to a beet, a vegetable with a naturally reddish color, serves as a vivid and relatable metaphor to describe the flushed complexion that can occur in such situations. It can also be used to describe situations where someone is confronted with unexpected attention, criticism, or humiliation.

Every time she speaks in front of a large audience, her face turns red as a beet from nervousness.
He always gets red as a beet when someone brings up his embarrassing childhood story.
I turned red as a beet when I realized I had been talking to the wrong person the entire time.
They blushed as red as beets when their secret was accidentally revealed during the party.
Tomorrow, she will turn red as a beet when she realizes she forgot her lines during the play.

used for describing a person whose face has turned very red because of hot temperature

The idiom "red as a beet" stems from the resemblance between the color of a ripe beetroot and the flushed or reddened complexion of a person. It draws upon the visual association of a beet's vibrant red color with the intense redness that can appear on someone's face. It is can be employed when a person's face becomes flushed due to exposure to hot temperature. The idiom highlights the intensity of the redness and is typically used in a lighthearted or descriptive manner.

After spending all day in the sun, his face was red as a beet.
I'm sorry, I can't stay outside any longer, I'm starting to turn as red as a beet.