When it is cooked tuna can be different colors depending on what your own personal preference is. There is nothing wrong with eating tuna that is pink in the middle as long as that is how you like to eat it. Just like with steak or beef there are different ways cooking it, such as medium, medium-well, well, and rare.
Eating tuna that is pink in the middle is not unhealthy at all and chances are there is nothing wrong with doing so. Just make sure that you know what your tuna is going to look/taste like before you order it a certain way at a restaurant or cook it at home. This way you will really be able to enjoy it.