Of all the artists out there mimicking the flow of paint, perhaps the greatest is Czech artist Ondrash, who can be found at http://www.facebook.com/ondrashtattoo or http://www.ondrash.com/ (which appears to be a work-in-progress so the FB galleries are your better bet for now).
I would go so far as to say that there is no one working today as capable at making tattoo ink appear like the subtle movement of watercolor pigment as Ondrash is. It’s quite remarkable, really. Whether the skin will allow these subtle nuances to hold up in the long term is of course still to be proven, but that is worry is true of a great many modern color artists, and only time will tell whether modern tattoo techniques and inks will allow for new expression in the art form that was not previously possible. Here is a small gallery of Ondrash’s remarkable work:
Finally, I want to emphasize how hard this is to achieve. The tattoo on the left is an original by Ondrash. The tattoo on the right is a rather callously and uncreatively stolen copy, and while the artist is obviously not incapable at his craft, it falls leagues short. If the wearer has any sort of eye, I am sure that every time they are congratulated on their tattoo by friends, that they not only feel a sting of shame, but also disappointment at how inferior their copy turned out.