Part of the social experiment of it is that it's supposed to be so minimalist that there's no need for instructions. That said, clicking a pixel is supposed to change it to your current color, kind of like in mspaint. What browser/device are you using?
Safari Mac doesn't work. Grey-on-grey checkers.
Chrome Mac does work.
I just tried it in Safari 11.1 on macOS 10.13.4 and it works. Grey checkers is when it's loading. It initializes the board by sending everyone about 10kb of data which on slower connections can take a few seconds. Other than that my only guess is that it only works on the latest version of Safary and Mac. I used lots of ES6 features natively in the browser with no Babel/compiling/etc phase.