Control a bicolor LED with just one pin
You can do this with a bi-color LED that has the two LEDs back-to-back if you connect one LED terminal to an intermediate voltage eg 2.5V on a 5V design and connect the other side to the MCU via a suitable resistor (I used 560R). 

Then a low output gives one colour, high gives the other and tri-state leaves the LED off. Pulse width modulation will allow you to control brightness (switching output between active and tri-state) or mix the colours (switching output low to high).

You can adjust the intermediate voltage to compensate for different LED forward voltages too. 

I used an LM2904 op-amp to provide the intermediate voltage - it works with supply voltage down to 5V. There are plenty of other devices that can operate at lower voltages and still sink and source enough current to drive the LEDs.