Caught red-handed -> Found in the act of doing something
Red tape -> (Too many) official rules and procedures
Paint the town red -> To have a wild celebration
Red rag to a bull -> Certain to produce an angry reaction
In the red -> To be overdrawn at the bank
Out of the blue -> Something which happens unexpectedly
Blue-eyed boy -> A person who is well liked and treated well
Blue blood -> Of noble birth
Once in a blue moon -> Something which happens very rarely