Colour
The use of colour is considered by many to be one of the most important areas in composition. Colours can be used in isolation or specific combinations to create particular moods or emotions. The theory behind the use of colour in composition is based around the colour wheel which is a visual representation of the spectrum of colours, with all of the visible colours mapped to a circle. Certain colours have particular moods associated with them, for instance red is considered a warm colour whilst blue is a cold colour.
Colour theory is a subject in itself and there is a huge amount of reference material available on the internet for those interested in pursuing the subject further.