I just came across this nice article – http://understandinggraphics.com/design/10-reasons-to-use-color/ – entitled 10 reasons to use color.

The article lists 10 good reasons to use colour in design. Number 10 is using colour for metaphor and taking advantage of the associations that are inherent in phrases such as feeling blue or green with envy. There is no doubt about the meaning in the image below; that the woman is filled with envy.