Colored Pencils is one of the easiest media to handle since they're pencils and you can control your strokes well. There are also a lot of available colors to choose from. First, cover the entire drawing with light strokes. Then, you can proceed with the thick lines to add texture and effect to your drawing. You can also add some more details in your drawing like this one. Soft pastel was used for the blue shadow_effect.
If you want a clean and bold look for your picture, you can use colored markers for the color. It has a wide variety of colors and you can choose between fine tipped ones and broad tips depending on your needs. There are also refillable markers where you can just purchase ink whenever it dries up or loses its color. It's also an easy material to use. You just have to apply a single coat on the drawing. You can also add some more details using either a white colored pencil or a white inked pen.