The primary function of a football facemask is to protect the player's face. That doesn't mean that you can't express your style while keeping yourself safe. With a bit of creativity, you could be taking to the gridiron while expressing your individuality. While you should definitely be careful not to do anything that would compromise the integrity of the mask, the only limits are those of your imagination. If your style is attractive enough, you may even convince teammates to adopt your look.
Examine as many different kinds of football helmet facemasks as possible. This will help bring you inspiration for your own design. In an article about the evolution of the football helmet, "Popular Mechanics" demonstrates that facemasks have changed a great deal over the years. Teams have also experimented with different colors and materials.
Brainstorm to get your ideas flowing. At this point, you should let your imagination run free. You could customize your facemask with a label emblazoned with your nickname. You could make the bars of your facemask look like a shark's open mouth for a more intimidating look. When it comes to colors, consider using some color theory. Donald Johansson, writing for Colors on the Web, points out that using complementary colors with opposite places on the color wheel (such as orange and blue) provides contrast, allowing you to stand out.
Finalize a design for your facemask by sketching a full-color rendering for reference. It might be a good idea to get feedback from others or set your design aside for a few days so you can look at it with fresh eyes before you start the customization.
Prepare your facemask for customization. First, clean the facemask thoroughly to make sure paint or other materials will adhere properly to the surface. Next, apply masking tape along the design you wish to add. This may take a little while, but it will help you put crisp, professional-looking lines on your facemask.
Paint your facemask using hobby enamel paint or something similar. Model-making enthusiasts use this to paint their metal models, and your facemask is made of metal, so this is a good choice. (It's not your only option, of course.)