You will always find someone out at the edge of body modifications, experimenting or trying something new just to see if it can be done. A much more rare body mod is starting to be seen more often but it's definitely not for everyone. What we're talking about here is tattooed eyeballs.
Most people's reaction to hearing this is "Oh my god, you can tattoo an eye?" And the answer is yes, although it comes with it's own collection of dangers and side effects. First off, when people talk about tattooing the eye, they are referring to tattooing the sclera, what is otherwise known as the "white" of the eye. As it is, tattooing the eyes should be considered much more risky than tattooing the skin.
The greatest dangers from eyeball tattoos is scarring and infection. Even the most simple bacterial infection can damage the eye and cause reduced vision or blindness for life, but the fact that eyes are right next to the brain also means that any infection that traveled into the body from the eyes would shortly be at the brain itself.
Colored tattooing of the eyes is very modern. Corneal tattooing actually used to be used in the days before contact lenses to permanently scar the corneas in an attempt to correct vision. In this day and age, laser correction is the modern version of that old process, and it works much more accurately and with less dangers.
So far the most popular option is to tattoo the whites of the eyes solid black. For people who have very dark irises, this can make the entire eye look dark, an effect done in movies with contact lenses or digital effects for various vampire characters or in the X-Files for people infected with "the black oil." But as eyeball tattoos have gotten more popular in the heavy body modification crowd some color is appearing. Some people have opted for green, orange or even blue (something that a very hardcore fan of Frank Herbert's DUNE might like).
While many of these first modifications have been done successfully, it's important to remember that educating yourself as best you can and seeking out someone who is an extreme expert is key to having such radical modifications turn out well. It's important to note that the "oldest" tattooed eyeballs are only about five or six years old at this point, and no one knows the exact long-term results of such modifications.