Patina
When metal meets weathering, naturally or not, it might take on a green or brown patina
Generally "patina" refers to the visible aging on surfaces over time, but in architecture it specifically refers to the color (usually green or brown) and texture added to certain metallic surfaces through weathering. The most typical metals in this vein are copper, bronze and steel. Patination occurs either naturally, through exposure to the elements over time (oxidization), or artificially, in factory conditions that accelerate the process.
The Statue of Liberty is the most famous example of a patinated copper; its recognizable green color covers every inch of the exterior. Architectural applications tend to be less sculptural, but nevertheless the green patina is still used for effect.
What looks fairly even from a distance is actually mottled up close. When materials like copper gain their patina naturally, the speed at which it happens will depend on orientation, climate and other factors. A soffit, for example, will take much longer to turn green than a roof or facade.
Patinated steel often goes by the trade name Cor-Ten; the material is coated with a layer of rust through weathering. The application is popular in modern buildings and landscapes. One needs to be aware that the rust can bleed onto adjacent surfaces, such as concrete sidewalks below a weathered steel wall.
Cor-Ten steel is fairly uniform, but it still has variation (in color and texture) that exhibits the process it underwent. A layer of rust may seem like a bad thing, but it actually protects the steel underneath, and therefore requires little to no maintenance.