The film used with the screen is blue/green sensitive so the red light that gets through a magenta screen has no effect on the image. I think the magenta was used for more speed in the exposure. I've used both magenta and grey screens and saw no real difference in how well they worked. One thing to consider is to look for an elliptical dot screen, these produce smoother gradations in the mid tones than a round dot screen.
Originally Posted by holmburgers