(as|) red as a beetroot

becoming extremely red in the face because of hot temperature

The idiom "red as a beetroot" is based on the visual resemblance between the deep red color of a ripe beetroot and the intense redness that can appear on a person's face. used to describe someone whose face has become highly flushed and red as a result of being exposed to heat or hot temperature. It emphasizes the intense redness of their complexion.

After hiking in the sun for hours, his face was red as a beetroot.
If you don't take breaks in the shade, you'll end up as red as a beetroot.

used to describe a person whose face has become very red because of embarrassment

I was red as a beetroot when the teacher made me speak in front of the whole class.
Oh, Mike definitely likes you—he turns red as a beetroot every time you're around!
She went beetroot red when her crush came up and talked to her.
Whenever I talked about his past life, he would go beetroot.