No matter how much art changes, color stays the same. There are only three primary colors — blue, yellow, and red. They combine to make the secondary colors — green, orange, and purple — which combine to make tertiary colors like blue-green and yellow-orange.

But how artists use colors is slowly changing, according to research by Martin Bellander, a PhD candidate in psychology and neurobiology at the Karolinska Institutet in Sweden.

Inspired by data visualization projects that tracked color use in movie posters and trailers, Bellander decided to do the same for paintings.

"Lately, I've been fascinated by color theory," he told me. "I wanted more data about artistic expression using colors from a longer period of time, and decided to look at oil paintings."

He used the BBC catalog of more than 200,000 paintings, created a program to scrape them for relevant information, and plotted their use of color into a stunning spectrum of the rainbow that shows just how much our relationship with color has changed.

This plot is based on 94,526 paintings Bellander analyzed, created between 1800 and 2000:

As Bellander's research shows, the use of blue has significantly grown over time. Bellander puts forth a few ideas for why this might be at his blog. He hypothesizes that the increase could be the computer registering dark pigments like black in the blue spectrum, aging resins in older paintings, or simply the cost of blue paint going down.

"Of course, this is just speculation, and these explanations don't exclude each other, either," Bellander told me. "Also, I think there might be additional analyses of the data that could start to delineate the influence of different factors."

Fine art has dramatically evolved in the past 200 years from portraits of kings and biblical paintings to Mark Rothko's large color theory squares and Jackson Pollock's messy floor. Blue isn't just for skies and water in modern art.

For more information on Bellander's methodology and work, check out his blog here.

(h/t Washington Post)