AHA! Signature color! You said it right there! Sparda's signature color is purple, Vergil's is blue, Dante's is red, and Nero's are red and blue. That's what I was saying - it does have symbolic meaning. They could've given Nero the color green, but did they? No. Why not? Who knows -
Nero does have Sparda's blood, but that doesn't mean he's Vergil's son. Seriously, why would he have to be Vergil's son? Sparda could've had relatives, who then had Nero. That means Nero has Sparda's blood too, since Sparda's relatives have the same blood as Sparda.
He could also be an experiment created with Sparda's blood, or that of Vergil and Dante.
Is it so hard to understand?