What we humans perceive as color is actually light waves that hit our eyes, translated into nerve impulses, and interpreted by our brains as all the various colors around us. Therefore, color not only involves physics and chemistry, but also physiology and psychology as well. In physical terms, however, what we call "light" (and subsequently, visible color) is only a very small part of a much larger phenomena. Visible light is electromagnetic (EM) waves. As you can see, the part of the spectrum that we can see (the colors) is only a very tiny part of the whole thing. Colors are distinguished from each other by their frequency. All red light of the same shade, for example, has the same frequency, while all violet light has the same frequency, but different from that of red. Red light has a lower frequency than violet light. In fact, the rainbow is arranged in order of frequency from red to violet.(The reason for this is explained in refraction.)