Idioms - Color - Blue

The universe is...big.
And things happen very... slowly.
Ah, the moon. A new moon is created... not very often.
In fact, it happens once in a blue moon.
In English, if things happen extremely rarely, we can say they happen ‘once in a blue moon’.

Once in a blue moon.

Like my students. They almost NEVER do their homework. In fact, they do it once in a blue moon. But they don’t really listen to me.
I say to them ‘Do your homework.’ I do, ‘Do your homework’. That’s what I tell them. ‘Do your homework.’ That’s what I say. ‘Do your homework.’
That’s right. I say it until I’m blue in the face.
In English, if we say something until we are blue in the face, we waste time and effort because we won’t get any results.

Until you are blue in the face.