That's the easy part, just use a color space with imaginary primaries (see e.g. ...

		ledoge 5 days ago \| parent \| context \| favorite \| on: NIST scientists create 'any wavelength' lasers 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.

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But that's still mixing just a few primary wavelengths.