One of the things I really like about drawing with a pen is being able to create interesting wash effects. I carry a waterbrush along with my sketchbook and pens. It is really very handy. Just a quick wipe and the ink bleeds and washes into some interesting effects and colours. Of course different inks react differently to water. It is something you need to experiment with.
In my fountain pens I use black Quink ink, black Lamy ink and Chocolate brown Diamine ink. With a lot of water the Quink washes out to some nice browns and blues. The Lamy stays fairly black and the chocolate brown is nice and warm with a red tinge. Gel pens will also wash but I haven't had much success with a biro, it dries too quickly.
I then tried it with the Chocolate brown Diamine but I think I got a bit excited and over sprayed. I do like the effect on the foliage in the background here, but I lost too much definition on the stone work of the barn.
This is something I will definitely experiment with further.