Put together by Vijay Pandurangan, the visualisation uses data from 35,000 posters stretching back to 1914, all the way up to the present day. Each image had its coloured pixels, and then all the posters were sorted by year and hue (click the image above to see a larger version). A designer friend of Pandurangan, Cheryle Cranbourne, gives a little insight into what the visualisation shows:
"[They] cover a good range of genres. Perhaps the colours say less about how movie posters' colours as a whole and colour trends, than they do about how genres of movies have evolved. For example, there are more action/thriller/sci-fi [films] than there were 50-70 years ago, which might have something to do with the increase in darker, more 'masculine' shades."
Other than that, feel free to read into the results what you like in the comments. You can also drill down into the data using an interactive version of the visualization. And before you ask: no, we don't know why 1924 is blank. [Vijay Pandurangan via Flowing Data]