Why does it matter when an author describes a certain thing in a certain colour?

An author might use colour to convey a particular mood or emotion. We intuitively associate colours with certain feelings; for example, red is often associated with danger or fear. By using this colour, an author is able to make the reader feel a sense of anxiety and impending danger in a subtle way. It is a subtle - but effective - form of descriptive language because, often, a reader might not even notice that the author's use of colour is affecting them in this way.