I have several red lasers, and two green ones: one is a Streamlight light+laser, and a pair of Crimson Trace grips.
I haven't done extensive side by side red/green testing. I haven't done so because my experience is that there are many lighting conditions where the green isn't useful outdoors. My summary would be that indoors, or outdoors in dim light, either red or green works well, and outdoors in full sun neither is useful.
I see that I'm out of step with most other posters, so let me elaborate. My definition of 'useful' means I can use the laser, quickly, on any background. At night or indoors across the biggest room in my house, for example, you can always see either laser immediately. In those conditions, the laser is always faster to acquire than the iron sights.
Outdoors in full sun, though, neither can be depended on to be faster than the iron sights. For example, if I'm sighting in a laser at the outdoor range, I make sure to be there at dawn/twilight in the winter, or a seriously overcast day in summer, and expect to have to hunt a little to acquire the laser at 25 yards.
It may be that there are times and backgrounds when you can see the green laser at 20 yards and the red at 10; I dunno. I just know that in full sun there are times when the green laser isn't useful at 10 or 15 yards (because you would spend longer finding the dot against green grass or a fuzzy sweater or whatever than it would take to use the iron sights).
My laser doctrine is to not expect the laser to work. If I've decided to shoot and happen to see the laser dot as I acquire a sight picture with the irons, then I'll use the dot, otherwise I just use the irons. That doctrine means that the dot has to be really obvious to be useful. In that context, I don't see a particular difference between red and green.
In other contexts - maybe plinking, or squirrel hunting, where one is more time to pick out a dot - then there may well be significant differences. I haven't explored those issues.
Or maybe my eyes are just different, I dunno. YMMV!