This is perfect! That makes a concentration parameter entirely unnecessary.
Here’s a question though: Could you perhaps optimize these things independently of RGB? On the full XYZ space? XYZ → Spectral would be the first goal, and from there it’d just be a matter to apply usual XYZ → RGB to get the right result, and it’d just go RGB → XYZ → Spectral to get everything as needed for what ever color space you might want.
Not sure if the horse-shoe outline poses any problem for that kind of idea. Clearly not all XYZ values would even be physically meaningful. But basically, you’d have to somehow work on this space
That’s right. I believe “the right way” to do this, in principle, would be to optimize a color over all possible spectral Metamers (under some reference lighting, perhaps Illuminant E) that produce it, finding the one with maximum entropy.
Though that is perhaps simpler said than done. I found some literature on this but as far as I could tell, it seems to always involve being trained on some data set of actual physical spectra, rather than theoretically considering all spectra that might produce a given color. And there might be a good reason for that.