The "color" of light is generally identified by the distribution of power or intensity as a function of wavelength.  Sometimes it is convenient to describe light in terms of "wavenumbers," where the wavenumber (w) is simply equal to the inverse of the wavelength and is therefore proportional to frequency.  

Wavenumbers are often used in Raman spectroscopy since the separation of a particular Raman line from the laser line is fixed by the molecular properties of the material and independent of which laser wavelength is used to excite the line.  This means that the shift is of constant frequency, regardless of excitation wavelength and can be conveniently expressed in terms of wavenumbers.

To learn more, please review our technical note Measuring Light with Wavelengths and Wavenumbers.