Let's whittle down your options some. Do you want to be able to use it in most ambient lighting conditions including outdoors or mostly indoors? If yes to the former choose green, if the latter choose red.
Aside from color choice what is your intended purpose, target, self defense, home defense etc?
Now, let's talk money, what is your budget?
The more info you can provide the more we're able to assist you...
Others may disagree but the only rail mounted pistol laser sight I recommend is a Crimson Trace. There are those cheaper and those more expensive. CT has been around a long time, they stand behind their products and they make quality products.
Just for clarification I am referring to a laser sight only not a laser/light combo.
Here's a Crimson Trace CMR-206 green laser sight. It's MSRP is $239.99, they are found cheaper.
Also, allow other members an opportunity to chime in, there could be something new and wonderful that I'm not aware of. Good luck!