I really can’t express just how much a simple set of grey Faber-Castell Pitt Pens can add to your work. It will add depth to your black and white drawings, and when pieces containing it are scanned, they can be color-shifted and maintain your hand drawn gradient. The packs contain “Cold Greys” and “Warm Greys,” which I usually alternate between judging by whether an object is inanimate or not. You can get them in “Brush” or “Soft Brush,” don’t get Soft Brush unless you have an exceptionally steady hand.
Take this sketch for example. It really only has one color (red), but the greys add enough depth to make it feel more “alive.”