Painting still life is a skill that requires practice to achieve a credible level of realism. Many beginners find paint metals and reflective surfaces as the most difficult part of making still lifes. You may be tempted to buy a tube of glossy metallic paint, but it will make your paint look fake. Instead, you can mix metallic colors like copper with basic colors and techniques.
Instead of trying to blend a copper color, you can mix the different colors you see. Look at the object and look for the primary color or the middle tone. The tone of this color is based on the environment and light of still life, so it is hot or cold. It starts with a neutral brown as natural shade and adds ocher yellow and cadmium red to make it a warm color. To get a cool color, you can add red cadmium and cobalt blue to yellow ocher. Try to clarify the color using mostly yellows and only use small amounts of white titanium. To darken the color, do not use black, better add dark blue and brown.
After finding the base color, mix and apply a color with low light in the area of copper where the shadow is. This low light painting is also hot or cold, depending on the environment. Mix red cadmium and cobalt blue to make a purple. Adjust the quantities by adding more red for a warm color or more blue for a cool color. Neutralize the purple slightly adding yellow ocher or bright Indian yellow. A neutral purple-gray accent will highlight the orange tons of copper.
The lighting points will make the copper stand out and give it an effect in three dimensions. Look for the color that is lighter in copper. The highlights of copper are usually very orange, so start mixing cadmium red, Indian yellow and some white titanium. To make the orange tone cooler, you can add small amounts of cobalt blue to neutralize it.
Most copper surfaces are very reflective, so if you have other still-life colors, they will look like copper. Look for any color in the copper that you have not yet mixed, such as greens, roses or blues. Mix a small amount of that color and add it to the base color to highlight the areas you think you may need.
Use a soft brush, like a saber, to apply the paint to the canvas. This will keep the copper surface smooth as metal. Apply the base color, then the low lights and mix in areas of reflected color. Add highlights at the last minute to keep colors from getting muddy.