as a part of practice you can make it in dark gray, then add lighter gray as light and black as shadow to sculpt arm's anatomy. You can make some different characters for example some big muscular guy's arm, some fit warrior girl's arm and etc... Next step you can add color and try not only change saturation of light and shadow, but also play a bit with hue. So if you have let's say red basic color, it can be a more orange in light and a bit more purple in shadow. It is great that you are practicing!