AnyBar
AnyBar is a small indicator for your menubar that does one simple thing: it displays color dot. What color means is up to you. When to change color is also up to you.
AnyBar is controlled via UDP port (1738 by default). Send it a message and it will change a color:
echo -n "black" | nc -4u -w0 localhost 1738
You can control AnyBar via AppleScript, too, or choose from a bevy of command line clients.
The app also supports custom images, and lets you run multiple instances, with each one listening on a specific port.