Colour idioms

RED

be in the red

If you or your bank account are in the red, you owe money to the bank

see red to become very angry

BLUE

once in a blue moon not very often

My sister lives in Alaska, so I only see her once in a blue moon.

blue-sky

using the imagination to think of ideas that do not yet have practical uses or make money:  Blue-sky thinking involves a group of people looking at an opportunity with fresh eyes.

out of the blue

If something happens out of the blue, it is completely unexpected:

One day, out of the blue, she announced that she was leaving.

GREEN

be green with envy

to be very unhappy because someone has something that you want:

Chad is heading off to Spain for the week, and I'm green with envy.

green fingers (UK) / green thumb (US) the ability to make plants grow

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once in a blue moon

green thumb