Key lime pie is yellow as is a ripe Key lime. If you see a green Key lime pie or green lime slices or meringue or whipped cream on a Key lime pie it is not authentic. In the late 1800's there was no refrigeration hence the condensed milk. There was no sour cream, no confectioners' sugar, no whipped cream, or any of the other fresh ingredients some people use to jimmy up their recipes.
The amount of juice you use is a matter of taste. I like to taste the Key lime, not the condensed milk so I use 1/2 cup of juice. If you buy bottled Key lime juice, make sure the ingredients say "100% Key lime juice" and not "Key West lime juice.
I use a Keebler's(R) crust, but it is probably better with a homemade graham cracker crust if you have the time and the energy. You actually do not even have to bake this pie, just refrigerate it overnight but it might slump a little. The citrus actually does the "cooking." Baking helps firm it up a bit and cooks the egg yolks.