Juxtaposition of Colour:
Every photographer needs to develop an understanding of colour if they wish to create successful images. This is even true of photographers who shoot exclusively in black and white, as different colours will react differently with your film and will result in different tones.
Colours that are complementary to each other will often be at opposite ends of the colour wheel, including red with green, blue with yellow etc. Depending on what you are trying to achieve with your photography, you should aim to use these contrasts of colour to your advantage, whether that means using colours which relate to each other harmoniously or jarringly.