Color is Light and Light is Color
Light energy travels as an electromagnetic wave with a characteristic wavelength between successive waves and frequency, which is the number of times a wave oscillates within a second. As first demonstrated by Sir Isaac Newton at the end of the 17th century, white light upon passage through a crystal prism, refracts and divides into seven component colors, spread out across a spectrum of varying frequencies, that is, each color has its own unique wavelength and frequency.
Red has the longest wavelength and on the other end of the spectrum is violet with the shortest. Beyond violet lies Ultraviolet (UV-ray), which is invisible to the eyes and has the ability to kill bacteria and can even penetrate the skin to provide us with a suntan effect. Beyond red lies infrared and then microwave, radar, radio waves, respectively, all invisible to the human eye.
Red, orange & yellow, exist on the longer wavelength side of the spectrum and are believed to reach the eyes faster than the colors on the other side of the spectrum. They are called warm colors because they are often associated with things such as sunshine, fire, etc. Green, blue & indigo on the other hand, have shorter wavelengths, are called receding colors because they take longer to reach our eyes, and are referred to as cool colors often associated with things such as the sky, waters of the seas & rivers, the winter dawn, etc.
Each warm color has its complementary cool color: red and blue, orange and indigo, yellow and violet, and lastly, green and red (magenta).