Working with a wet brush onto a drawing done with a pen containing water-soluble ink turns the line into a wash of ink. Depending on how much water you use, more or less of the line dissolves.
Quite what color wash you get depends on is in the ink; it's not always what you might expect, especially with cheaper pens. (The potential problem with using a cheap pen is how lightfast the ink might be, but they're great for experimenting, just keep the results out of direct sunlight.) In the example in the photo I was using a black marker pen I bought at a supermarket on a whim, a black Berol handwriting pen. As you can see, it's "dissolved" into two colors, a result that I think is appealingly effective and expressive.
Quite how water-soluble a pen might be depends on the brand, but the starting point is to look for one that doesn't say "waterproof", "water-resistant", "water-resistant when dry", or "permanent". How long the ink has dried on the paper can also be a factor; some waterproof pens will smudge a bit if you apply water immediately.
Buy Direct: Lightfast, Water-Soluble Ink Pens