I like to use different pens for different things. I use thick markers for large shadows because they’re big and chunky and cover a lot of paper quickly. I like Sharpies for smaller shadows and thick continuous lines. A blue writing pen might be good for clouds, waves, or anything that might look cool in blue. Consider all characteristics of your pens, like colour, thickness, etc. Be creative and stay alert for any pen that can make your work distinct.
