Quote Originally Posted by Ed Sukach

I decided to analyze the gelatin filters 0 - 5, using the ColorStar 3000 (that took some thought), and discovered that they were *not* pure yellow or pure magenta; each transmitted a combination of yellow and magenta.
Very practical! Not surprising this fact, I believe it's done to keep the exposure times more consistent when changing contrast filters. I've seen a website dedicated to calculating values for your own system using this mix of yellow and magenta, creating a neutral density of sorts to buffer the exposure time as the proportions change.