Vijay Pandurangan has considered these colors along with those of many other films. Below are his findings for the year 1989:
But he has done more. He has looked at the colors in movie posters from 1914 to 2012 and produced an interactive image where you can select any year within this range and see the pie chart of movie posters from that year. Here is a still image of his interactive one.
He also produces an interactive image (still image below) with lightness and saturation ignored.
Movie posters seem to have gotten bluer over time. We've seen movie poster colors here before. Thanks for the link Nick.