This would be for artists.

It is a pen with three ink tanks, a red, a green and a blue. You would could change the ink mix ratio coming out of the pen, and make its trace any color you would like.

Apparently people tried this a kickstarter, and it wasn't sucessfull, to my knowledge. I'm afraid the reputation of this idea got a little tarnished after that.

In the first design, three flexible parallel tubes would go from the tanks on the top of the pen to the tip. There, the ink would mix before being transfered to paper by a sphere, like in a ballpoint pen. There would be three screws compressing each one of the tubes, in a way that when loosened, they could control the proportion of each color.

In a second design, more complex, the screws would be inclined, say 75° towards the body of the pen, and closer to the tip. They would not compress a flexible tube in this case, but would have magnets on their tip, inside the pen. These magnets would be close to a sphere, somewhat larger, and the ink would pass though the laterals of the sphere, between the sphere and the pen border.

In the rest position, the three spacings where the ink would flow would be the same and the same proportion of ink would mix: the pen is black. When you loose one of the screws, the sphere is displaced, closing more one side while opening the other, what changes the proportion of ink, and hence the color of the pen.

I suppose the first idea is better now that I've written this.