Everyone knows that red means danger, but how did purple become a cautionary color? At an EPA conference in the late 1990s, attendees nearly came to blows over color coding on the Air Quality Index.

When it's more than 200, it's purple, very unhealthy. This makes sense to information designer Giorgia Lupi. GIORGIA LUPI: From yellow to orange to red and purple is the next color in the spectrum.