This little experiment made by Vijay Pandurangan started as a challenge to demonstrate his friend that movie posters these days were getting darker than years before. To demonstrate his hypothesis, he went on to collect all the data he could find of movie posters from 1914 on, creating color schemes for each year.
His methodology consisted of downloading thumbnailed-size images and grouped them by year, for every one of them he counted the total of pixels-per-color for that year to finally create the entire visualizations. There are two versions of the full time lapse graphics, one that considers lightness and saturation and the other one that doesn't, thus creating a "clearer" presentation of the full data.
So apparently the hypothesis was on the right track. We can see that the brighter graphics were more popular in the start of the 20th century while darker hues start taking over the color palettes in the last few years, which makes sense considering the overbearing presence of orange and blue in movie posters.