What is colour: Colour is simply light of different wavelengths and frequencies and light is just one form of energy that we can actually see that is made up from photons. we are all surrounded by electromagnetic waves of energy of which colour is just a small part.
Why use colours: We are in a world where colour dominates our lives, from reading signs on the road to seeing if fruit is ripe to eat. It affects our moods - blue is calming - red can make us tense. We use and experience colour everyday in our lives without even appreciating it. Colours does a lot of useful things to help communicate information. First, it attracts people's attention. Hotter, brighter colours attract attention more, and cooler, darker colours attract it less. Secondly, people tend to relate similar colours and separate colours that are different. You can use this in your graphics to help people relate similar ideas by colour coding them. Thirdly, colour can help people learn what something looks like so they can recognize it again. Fourthly, most people like colours and simply enjoy the presentation more if you use it.