The DCT of wavelengths that create the non-spectral colour.
No clue how you'd film in this format but the CGI and video games would be epic.
That's the easy part, just use a color space with imaginary primaries (see e.g. ProPhoto RGB), or use one with real primaries that allows for negative values – e.g. Windows uses floating point scRGB for HDR, which is just linear BT.709/sRGB, but negative RGB values can be used to cover the full range of real and imaginary colors.
But that's still mixing just a few primary wavelengths.
Just save pixel values as wavelength rather than RGB?