Colours. I've always thought of them as just something to give an image some vibrance. After going for this lecture, I now think that you can probably tell a different story using the same image but different colour combinations. I think it's just how our minds perceive these colours. i.e. red conveys strong emotions as it come across as being very attention grabbing.
blue on the other hand, does not have the same effect.
Furthermore, it's not just the meaning of the colour itself that is important, but rather the context in which you use it that sends the nonverbal message.