Aspect of any object that may be described in terms of hue, lightness, and saturation. It is associated with the visible wavelengths of electromagnetic radiation, which stimulate the sensor cells of the eye. Red light has the longest wavelengths, while blue has the shortest, with other colours such as orange, yellow, and green between. Hue refers to dominant wavelengths. Lightness refers to the intensity or degree of shading; it corresponds to the subjective sensation of brightness. Saturation pertains to purity, or the amount of white light mixed with a hue. The colours red, yellow, and blue, known as primary colours, can be combined in varying proportions to produce all other colours. Primary colours combined in equal proportions produce secondary colours. Two colours that combine to form white light are said to be complementary.