Something I like to do to create a series of illustrations or just sketches is to use the same colors / same paper for a group of drawings. They don’t even need to be all about the same subject matter if you are linking them with other elements.
One of these experimental series was done with a blue brush pen on brown recycled paper. I made a couple of portraits with pencils, inks, blue colored pencils (as shown above) and really liked the look of the materials on the brown paper. I decided to go for a nice solid look with the blue ink and I think it was successful.