coulours in idioms

 to be in the red = to be in debt; to be overdrawn at the bank. The opposite is to be in the black ( = to be in credit at the bank)

   green = inexperienced, not very competent

  to scream blue murder = to shout very loudly, to   

yell

 to be caught red-handed = to be caught committing  a crime  

out of the blue = completely unexpectedly

 to have green fingers = to be a good gardener, to   

be able to make plants grow well

 to paint the town red = to celebrate

 to do something until one is blue in the face = to do it again and again without getting any results

 once in a blue moon = very rarely, hardly ever

 to see red = to be really angry, really mad

 little grey cells = brains