what colors go good with copper trim

It will slow oxidization initially, while also removing the surface shine/glare, but no finish can prevent it.
Actually, if you don't wipe fingerprints right away from clear finished copper they will show even more, as you can't polish them away later, like without any finish.
- The acid from the finger oils creep through the finish and then trap it right on the metal.

Copper outdoors, in humid conditions especially will eventually tarnish/oxidize to a green dull coating, which protects the copper actually, as used on historical buildings and churches for ornamental roof covering all over Europe.

On aluminum a similar but much quicker process is used to 'anodize' it using acid and electric current, protecting the aluminum surface over time.

To retain a fresh polished metal look, only polishing with the correctly designed medium and applied method will give the desired results.

Several companies offer their products, but none can retain the fresh polished look for as long as they like to advertise and sell the product.

 

Btw: I have used copper, nickel, chrome, aluminum, gold, brass, steel and stainless metals, both plated or pure, polished or brushed, oxidized, antiqued, powder coated or plain clear finished ... for my tube amplifier chassis over the years and had to learn how to keep certain looks and appearances.
And this was for indoors applications with mostly finger acids/oils attacking the metal surfaces.
Metals used outdoors is something you have to read up and experiment and prepare for.