I remember one or two years ago, I bought a two color LED from Radial (Radio) Shack. I used my PC's parallel port to turn it on and off/flash it. I added a diode and some capacitors and the LED would fade on and fade off. It reminded me of a radio tower. If I wanted to use the other color, I just had to reconnect it in reverse. A bi-color LED is just two LEDs connected in reverse-parallel.
Not too long ago (months), I figured out how to use a PICAXE to do the color changing for me. I overcomplicated it and ended up using 4 diodes.
Not too long ago (minutes), I found a way to do the color changing using only analogue parts. I'm going to share it with you.
Step 1: Parts Needed
1. A Bi-Color LED
2. a 9V battery
3. a pack of battery clips
4. two resistors, each one appropriate for the two colors of your LED
5. 4 diodes
6. a DPDT switch
7. a pack of alligator clips
9. sweat rag (optional)
10. SPST switch
Step 2: Connections to the Battery
The DPDT switch has 2 common connections, both should be in the middle of the switch, we directly connect the battery clips ground to one of these. Then connect the battery clips positive supply to the SPST switch, then the other terminal of the SPST switch to the other common on the DPDT switch.
Step 3: Shorting Terminals
Looking at the DPDT switch from the bottom, all terminals going up, use something conductive, preferably wire/alligator clips, to make the following connections:
top-left terminal to bottom-right
top-right to bottom-left
Step 4: Adding Diodes and Resistors
Grab 2 diodes and connect them in series. Do this with the other two diodes, but keep the setups separate. Taking one setup, treating it as one diode, connect one of your resistors in parallel, connect both ends. Do this with the other setup.
Step 5: Finishing Up
Remember treating two diodes as one? Now it's time to treat them as 2 again. Where the two diodes meet on one setup, connect those terminals to A3/B1 on the DPDT switch. On the other two diodes, connect the point where they meet to A1/B3 on the DPDT switch.
This is very important.
You have to connect the LED between the resistor and D2 (shown in the picture). And if you did what I did in the picture (use two different resistor values), you have to connect the LED the correct way round.