[Tooster]

I might be misunderstanding parts of the comment above, although I think it aligns with what I had in mind. Here’s what I meant:

If a ray carries full spectral information, then a transparent material can be described by its absorption spectrum — similar to how elements absorb specific wavelengths of light, as shown here: https://science.nasa.gov/asset/webb/types-of-spectra-continu...

In that view, transparency is just wavelength-by-wavelength attenuation. Each material applies its own absorption/transmission function to the incoming spectrum. Because this is done pointwise in the spectral domain, the order doesn’t matter:

OUT = IN × T₁ × T₂ (or in a subtractive representation: OUT = IN − ABS₁ − ABS₂).

So whether one material reduces 50% of the red first and another reduces 50% of the green second or vice verse doesn’t change the result. Each wavelength is handled independently, making the operation order-independent.