Spectral rendering for refractions

[fixeighted]

Ah...I summoned Lele and he beat me

[fixeighted]

Oops, missed Oliver's eureka moment also
Seems non-obvious as a solution but I'm sure there's sound reasoning behind it...

[^Lele^]

Fog is a filter.
You see red in the top glass because green and blue are absorbed by it.
The green from below will never come through if the color is pure red, or set to absorb it all.
The plus operation of the overlayed glass, in other words, comes after the filtering by fog/refraction color.

[fixeighted]

I figured it was an operation of some sort but got there myself by tweak/observe method. The explanation is welcome, thanks