Whether or not these colours (shown in the image below) are primary colours doesn't really matter here, but primary colours also have different wavelengths and frequencies respectively, in the visible spectrum of electromagnetic wave.
However, the image shown above uses three different colours to represent three different frequencies or wavelengths.
Wavelength is inversely proportional to the frequency. f = v/λ, where v is the speed of the wave, f and λ represent frequency and wavelength of the wave, respectively.
And here those colours represent different wavelengths of electromagnetic waves, - red carries the longest wavelength with the lowest frequency, while blue carries the shortest wavelength with the highest possible frequency. Green carries between them.
