There is no copper plating under electroless nickel.
Electroless nickel is not a electroplated finish so it needs no under coat.
In any case, many guns don't have a copper underplate.
Like all gun finishes, when Electroless Nickel gets worn or damaged there's nothing that can be done short of a total refinish.
All you can do is clean the metal thoroughly to remove any surface dirt and what you get is what it is.
To clean you can use most any solvent, like paint thinner, lacquer thinner, or just apply a coat of CLP Breakfree and let it soak a day or two to dissolve any dirt or crud.
Once you have the surface dirt off, what you see is just going to be the way it is unless you have the gun refinished.
Using metal polishes will only thin or remove the thin surface coating. The gun may look better, but you will have done serious damage to the finish.
When it comes to scratches or just worn away finish, "That won't buff out". If you try, you also polish away the remaining finish.
If you'd like to have it new looking, Colt will refinish it for you.
They're not taking refinish work until August, but they can make it look like a brand new gun.