Colour originates from light and different colours have different wavelengths. The eye perceives colour when it receives reflected light from object surfaces. There are two main colour systems - the additive RGB system used in screens mixes red, green and blue light, while the subtractive CMYK system used in printing mixes cyan, magenta, yellow and black pigments. The colour wheel is used to visualize hue, saturation, tints, shades and complementary colours. Colour schemes like monochromatic, analogous and complementary create visual harmony. Colour perception can be influenced by surrounding colours.