White is obviously best for "all-around" use.
Blue, as stated above is good for tracking wounded game. Blood shows up as dark black under blue light. Blue is pretty bright to human eyes, but it's also a bit harsh. Blue also has a fairly short wavelength and may have some slight blacklight effects, such as fluorescing highlighters, but it probably won't show UV ink or markings (like on IDs) very well, if at all. Use a violet (405n) or UV (395-375nm) light for this.
Red is excellent for preserving night vision. The reason for this is that it doesn't destroy the rhodopsin in the rods of your eyes. Rhodopsin is how we detect light; it's constantly being built up and destroyed, which is why you see spots when you blind yourself with a bright light (bare areas of rhodopsin), and why it takes a while to get night-adjusted (it builds up and fainter lights like dim stars can be seen better). Red doesn't destroy rhodopsin. However, red is still visible by the cones in our eyes, which detect color, so we can see by red light. Many game animals such as deer cannot see red, so hunting at night can be aided with red light without spooking game.
Human eyes are most sensitive to light at around 530nm wavelength, which we see as green. Thus, of all the LED colors, green appears the brightest to us, and thus is very effecient to make a bright green light. Green also doesn't screw with NVDs (night vision devices), but that only really matters if you're SWAT or Spec OPs or something (or just a NV enthusiast). Green however doesn't allow very good color rendition.
Yellow green is kinda an in-between for green and white for better color rendition.
Personally, I would not see a normal, every day use for anything but white or red LEDs on an A2. I've thought about giving an A2 to my dad, who's a major astronomy buff. He goes through red flashlights like crazy; god knows how many minimags (and even more bulbs and batteries) with red filters he's gone through. The nice thing with the A2 is that you can lock out the incandescent mode by twisting out the tail cap, so there's no chance he'd accidentally blast himself or somebody else with 60+ lumens of white light when he only wants to read a star chart 2 feet away with red light. The only thing that's stopped me from buying him an A2 is the fact that he likes to lose things.
I'm planning to get a Surefire A2 and am wondering which color LED's I should get. What are some pros/cons on each color? ***I have found many of the A2 posts very informative*** but i still can't seem to make up my mind.
