"<...> pure colour forms using Dan Bruton's Spectra approximation<...>"

Approximation or not, you cannot assign a unique wavelength or frequency to perceptual (extra-spectral) colors if for no other reason than that the given λ(f) is factually incorrect. Moreover, even an approximation doesn't make much sense, as simply human vision perceives magenta as a very different color to say various shades of pink (white + red) or any other color near red [orange. orange-red, etc.]. Basically, there is no single λ(f) - spectral - color that looks even vaguely like magenta (similarly for cyan, 'fuchsia' etc.).