Make sure you wear a proper industrial mask and some lined plastic gloves that can handle being exposed to acid. Also do not attempt this project indoors!

Take Muriatic Acid and either add it to a spray bottle or dump directly onto the galvanized metal. It will eat away at the shiny coating very fast.

Use a rag to wipe around the Muriatic Acid. You will see the metal loosing it's shine quite quickly. 

Use a combination of black and grey spray paints. Lightly spray the container from approximately 18 inches away so that you are only lightly applying colour to the galvanized metal container. 

You can either be done at this point or you can go over the container again with a bit more muriatic acid. If you got too much paint in areas then you can remove it with the Muriatic Acid as well. 

Clean the container with mild soap and water to stop the acid from reacting.