Richard;
A paper and negative can be made that will not tolerate any other film/paper combination. An example would be a film with a cyan dye at 650 nm and a paper with sensitivity there. They are matched. A film as above, printed on a paper with sensitivity at 700 nm would show crossover.
Fuji paper generally has a shorter and more narrow red and blue sensitization than Kodak papers. As a result, they tend to produce crossover with some films that do not match it perfectly. Kodak OTOH, uses very broad sensitizations to make it work with all possible films.
Now, for filter pack. Since all emulsions are blue sensitive, and for stability reasons and speed reasons, the yellow layer is on the bottom of paper, not the top, then all layers can see blue light and therefore they can form cyan and magenta dye based on blue exposure.
To solve this problem, Kodak has used a blue layer which is at least 1.5 log E faster than the other two layers, and a magenta layer which is at least 0.6 log E faster than the cyan layer. This has three results. It renders the mask colorless to the paper, giving the paper a higher relative speed. It allows the use of only M and Y filters, and last but not least, it filters out the blue sensitivity of the C and M dye forming layers.
So, a paper yellow will be purer the higher into the red the filter pack is, and will be more of a pumpkin shade the lower the red in the filter pack. The ideal filter pack of course, is achieved with tricolor printing which gives the colors with the most purity.
Kodak paper is optimized at about 50R.
PE