Are you feeling blue today? Perhaps, if your business is in the red, there's plenty to feel blue about. In this sentence, red is a symbol of danger, meaning that the business is on the brink of closing or crashing. Feeling blue is another way of saying your sad.
Since colors are so closely related to emotions, using color idioms can make your language more powerful and well, "colorful" (intriguing and diverse). For example, if someone is not brave. Someone might refer to them as yellow. This is generally used when someone refuses to take on a challenge due to cowardice. A friend or foe might rhetorically ask them, "What? Are you yellow?" as a way of provoking someone to due something.
你今天感到忧郁（blue）吗？如果你的生意处在”红色”当中，那你就很郁闷了。在该句中，“红色”是危机的象征，意思是生意处在歇业和倒闭的边缘。Feeling blue 是表达悲伤的另一种说法。