Yes, you are correct. Apology. But the principle is the same. The yellow filters pass green light but not blue. The magenta filters pass blue but not green. One has to be careful with colours. We see a blend of pure red and pure green light as yellow, but we also see pure yellow light as yellow. Our visual wiring is such that we cannot distinguish between the two ways of getting the same result, although a photographic paper might be a very different beast in that sense. Which is why I said one must have the spectral response of the paper at hand and compare with the transmission of the filters to see where they overlap. How the three-layer paper differs with respect to two-layer paper would be interesting. Maybe it allows them to make the low-contrast layer even lower contrast, while a mid-layer makes it a bit more controllable with a straighter, more gradual transition between grades - I don't know. What I do know, is that it works wonderfully, and saves my @$$ when I expose or develop carelessly, which happens more often than I would like to admit .
Originally Posted by Michael R 1974