Fun post! I like reading blog posts like this.

I did wonder about one thing:

> The first measure I tracked was a ‘Colourfulness score’. This measures how intense and varied the colours are in a poster. Higher values mean the image contains more saturated and diverse colours, while lower values indicate a more muted or monochrome palette.

As the description suggests, it seems to me these are actually two things: one is how saturated the colors are and one is how diverse they are. In other words one is how much color there is and one is how many colors there are. The examples given for each color later on are ones where a single color predominates, but it's not clear from the article how common this pattern is.

Personally I'm intrigued by posters that use a wider palette. Subjectively it feels to me that movie posters have moved towards a minimalist design over the past 10-20 years (as have book covers), making single colors more central, but I'm not sure whether that hunch would be borne out by data.