Wheat Field, Vincent van Gogh; Reed pen and logwood ink over pencil; roughly 9 x 12 inches ( 24 x 31 cm); in the collection of the Metropolitan Museum of Art, which has both a zoomable and downloadable image on their site.
I love these pen and brown ink drawings Van Gogh did late in his career. I feel like he was trying to express color monochromatically. You can actually think of his ink as multi colored in the way he as used it with different size strokes and what looks to me like the addition of small amounts of water.
I think of these effects as highy sophisticated. When you look at the work from early in his full time period as an artist, how rough and basic it was, and compare it to the work in his later years (less then 10 years later), the progress he made as a determined self-taught artist is remarkable.