"Each user could choose one pixel from 16 colors to place anywhere on the canvas. They could place as many pixels of as many colors as they wanted, but they had to wait a few minutes between placing each one."
Here's a time-lapse picture of how that played out over a period of 72 hours:
This is the most fascinating art-and-sociology project I have ever seen. The link above explains how factions emerged and coordinated, focusing on just a few of the things that happened (like how that American flag ended up in the middle).