RED

be in the red

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

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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.

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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.

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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.

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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.

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

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