I was struggling with using markers at work. We are pretty restricted with stationery, black and red Sharpie markers mostly, so I was trying to figure out a way to deal with medium tones. Some things need to be softer than stark black and white. So I went and bought some grey Copics to allow me to explore more dynamic shading. Worked out ok. They aren't the best for large area shading in a hurry, but it allows me to get a softer edge. I may need to get a couple more, possibly darker grey, to really create nice gradients between light and shadow.