I came across a pretty big collection of these while I was in school. They're pretty much watercolor illustration markers, which you see used a lot in concept art. Especially car and exterior design like these.
Some of my favorite concept artists like Syd Mead and Ralph McQuarrie used these from time to time amidst some airbrush and digital touches. With the rise of PhotoShop and Painter, these markers probably don't come in as handy, but they're still great for getting a unique watercolor look like this.