The idea is, that in a panic situation, alignment of the two-colored sights is faster and easier.
Ive heard this a number of times, and I have to disagree. Perhaps if youre a total novice, and dont have any time in with practice and shooting you might have an issue, but if youre accustomed to presenting the gun and shooting, its pretty much impossible to screw it up. You actually have to work at getting them out of alignment, and the gun feels totally wrong in your hands when you do, and the sight picture doesnt look right.
Aligned, they look like this...... ***
Misaligned, they look usually look something like this.....off to the left... * _ * * or off to the right.... * * _ * (ignore the _ it wont let me space them right without it)
Ive used three dot sights since before they were lit, and have never had any issues using the dots and getting proper alignment and getting good hits, stress or no stress, bright light down to no light.
Once your brain is wired to the dots, you actually pick up the dots before you get a traditional sight picture, and that front dot is what you lock onto first as the gun comes up. Put that dot where you want the round to go, and as the gun rocks into presentation, the rear dots naturally align and give instant confirmation of your alignment.
You can actually shoot very well with them slightly out of align and slightly below your line of sight. You get a small, somewhat shallow triangle with the front dot at the top that points right to the target.
From what Ive seen, the green sights are the brightest and longest lived of the colors. They also usually offer the longest warrant (12 years), for them too.