depends on if the guac is on the top or on the bottom of the 7 layer dip. If you put it on the top then yes it will turn brown, unless you squeeze some lime juice on it and add 2 layers of plastic wrap on it. I've done this many a time and it turns out well. One layer of plastic wrap should touch the guacamole (no air pockets), and one should cover the entire dish. But to be extra safe, you can always put the guac on the bottom
This is the order that I use and I have never had brown guacamole. The secret as bitsubeats says is no air pockets. The guac must be totally sealed.
Sour cream mixed with 1 Tbl. taco seasoning
Some people like black olives, I don't.