Approach (yellow) signal indication

Applying the proper AC voltage should give a yellow indication. The +/- polarity will flip-flop fast enough that the red/green will "combine" to appear yellow.

Edit: This will obviously complicate the wiring but the results just might be worth it.

Operating multiple signal heads on one mast with 5V operation

What if you want multiple signal heads on a single mast, and you want them driven from digital logic? Yes, it can be done. What you have to do is generate a pseudo-ground signal at 2.5V and apply it to the signal mast. Then each LED has a single wire running down the mast, and if 0V is applied, the signal shows (as might be) red, and if it's 5V, the signal shows green. Use an alternating voltage and you'll see an approximately yellow color. It works!

Great Articles - one small nit

Great article. I like the use of common materials and re-purposing holiday items. I appreciated Mike Dodd's driver circuits but I believe there was one small error in his Fig 5. I believe he  meant to show both the pull-up and voltage divider resisters tied to +V at the top of the drawing. Or did I miss something?