I'm going to ramble a bit about traffic lights and see if something interesting falls out. I briefly thought about Unicode, but its not nearly well supported.
Traffic lights convey their signals through color.
I briefly thought it would be neat to convey Perl grammar via color, but that can't be done for similar reasons to Unicode. The interesting thing about traffic lights is the color is just for clarification. The real communication is through position. If you ask anyone on the street what 'stop' and 'go' are on a traffic light, they'll probably say 'red' and 'green' without even thinking. But if you asked them to draw a traffic light they'd be sure to put the red on top and green on bottom.