The Facebook Color scheme is set to blue because Mark Zuckerberg suffers from red-green color blindness.

 

According to The New Yorker, Zuckerberg is red-green colorblind, which means the color he can see best is blue. That also happens to be the color that dominates the Facebook website and mobile app.

"Blue is the richest color for me," he told the magazine. "I can see all of blue."