Lots of paint on the lumberyard shelves have copper or zinc listed as an ingredient. The percentage of actual metal will be listed on the label, in the ingredient section. Watch for any indication that the product had a certain percentage of active metal before dilution. The quickest way to see if a product might really stand up to the claims it makes in its advertising, is to look at the label and see what the percentage of solids is vs. the inert ingredients. I like the natural look.
Pigment is the primary ingredient that protects the wood from ultra-violet rays.
When you see wood starting to gray, you can be sure UV is damaging it. When UV hits the wood it starts being absorbed by the lignin, which is the material that binds the wood cells together. Once destroyed, the lignin washes out of wood, exposing the cellulose fibers, which are white. This is why the wood looks gray. They do, at first, but the infrared rays from the sun draws out the oils and resins.