Key: “concepts of selective and nonselective neutrals are of great importance in understanding many aspects of color photographic theory and practice.”

The curvy stuff adds up to the same grey, but will not look the same.

[This is why we used more than one type of film to make separations from. At least partially why.]

Not all should be linear, if you are after more impact of color — an increased color response comes from understanding the more that additive mode.

[ this is from Evans… my bib page ]