Although this link shows Lens 1/96, the colors still apply.
http://www.modelwarships.com/index1.html
If by "keel" you mean the rudder, then you can either copper it or leave it wood or black in color, it has been done both ways in the years past.
I also have this kit and plan on building it next and I will be following this color scheme.
http://www.historicships.com/TALLSHIPS/Shipways/ShipwaysMenu.htm
The USS Constitution has been around for along time and has had many alterations to her details. Best to research and find the version you like the best and go with it.
Lon