Depending on how pale your skin is, you can get weird results. On of my wife's tattoos was done entirely in white ink, and the problem is that after a tattoo, skin heals slightly unevenly. With most inks you can't tell, but with white, you can see the individual layers where healing has taken place. She ended up redoing bits of it in orange and yellow.

Of course, there are the fluorescent inks, but they all contain mercury - pretty much a BAD THING!!