Actually, you are starting from the wrong end of the problem, but let's do it your way and then get down to brass tacks. Without question, green lasers are more brilliant than red ones. I have three handguns fitted with laser sights, all red. At the indoor range I belong to, which has chopped tires as backstop, I can just make out the red dot when I shine it on that black backstop twenty three yards downrange. Another fellow a few stalls down lit off his green laser and the reflection off the same backstop lit the entire place up and almost blinded me in the process. Our eyes are much more sensitive to green than to red. So, if your intended use is busting bowling pins at seventy yards in the noonday sun, green is your choice.
That's not why I have three guns fitted with laser sights. I am old, and have them for home/self defense. I practice with two of them more or less weekly at five - seven yards at an indoor range, shooting at silhouette targets. I want four shots in the center of mass as fast as I can get them off followed by a fifth in the skull, and that is what I usually do. At this distance, a green laser may create more problems than it solves -- for example, it will light up a typical room almost as bright as day. Furthermore, every green laser I know of is awkwardly mounted on the handgun (which makes it almost impossible to carry in a holster) and clumsy to use.
My handguns wear Crimson Trace laser grips. As the name would indicate, they display a red laser beam, and come with their own grips, which in my experience are better than the ones they replace. They are easy to zero (and hold that zero well) and are instinctive to actuate and deactivate. And just about any holster that your handgun fits in now will not even notice the tiny change caused by the Crimson Trace grips. I cannot make any of these statements about any other laser sight I know of. Crimson Trace products are unique.
Crimson Trace makes sets for many Glock models, but dunno about your Glock 20. They are always adding new products, so stick with them.
Hope this helps,
Jack