This might be because of two color processes (e.g. two strip technicolor or cinemascope) which were cheaper and excluded either yellows or blue hues. Purple (or

This might be because of two color processes (e.g. two strip technicolor or cinemascope) which were cheaper and excluded either yellows or blue hues. Purple (or magenta for bright spots) also is complementary to green so they tend to be used together. Personally I always associated sickly yellow and greens with horror, but that seems to have been a trend perhaps.