Evidence of the Stark blood....just because he was born to Rhaegar doesn't make him less of a Stark. Not to mention Ned NEVER calls Jon his son, he always refers to him as "my blood" or "the Boy" or something. And remember that Ghost, which is considered Jon's "2nd half" is all white and has red eyes, much like Targayens.
It's also part of the reason why i think where Howland Reed is is such an important angle...at this point he's now the only living person that knows what really happened at the tower of joy.
Also it makes sense for Jon to be the prince seeing as how if you look at the overall crux of the story Jon is the only person fighting against the REAL enemy in the Others.
didn't think about about but I thought the Targayens had purple eyes?