The color spectrum is the entire range of light wavelengths visible to the human eye. These range from approximately 400 nanometers per wavelength, at the violet end of the spectrum, to 700 nanometers per wavelength, at the red end of the spectrum.
Spectral colors refer to those colors that contain only one wavelength. These "pure" colors were initially divided by Sir Isaac Newton in his studies of optics into: red, orange, yellow, green, blue, indigo and violet. Other colors are mixtures of these pure colors, and as such, contain multiple wavelengths. Further research has suggested that indigo is not a pure color, but rather a blend of blue and purple.