look blue - иметь унылый вид
Don't you know why he looks blue?

to sail under false colours - обманывать, лицемерить
Although he assured us of his support but he actually sailed under false colours.

once in a blue moon - очень редко
Once in a blue moon we decided to go on vacation abroad but his leave was cancelled.

out of the blue - совершенно неожиданно, как гром среди ясного неба
He lost his job out of the blue.

to look through blue glasses - смотреть мрачно, пессимистически
Her mother looked through blue glasses at her career.

green hand - новичок, неопытный человек
It is not a job to be handled by a green hand.

catch (someone) red-handed - поймать (кого-то) с поличным
The children were caught red-handed in the garden trying to steal apples.

white elephant - бесполезное приобретение
The new bookkeeping program was a white elephant and nobody wanted to us.