Common sense would point to Grey.
I think Black and White is a cool theme. Very stripped down, back-to-basics style. The most simple and classic contrast of all time.
Speculation: since Black is commonly associated with darkness and White with light, and we have a Dark type, this could be the introduction of a Light type. A Dark type legendary for Black, a Light type legendary for White.
Jack Of Clovers' idea of yin/yang legendary Pokemon seems likely as well, although they may not work in harmony but could clash like Groudon and Kyogre, with a third legendary to balance the two (who will be the theme of the 3rd version).
It's possible that other Pokemon could also be part Light, like Mew or Arceus. The balance of types is pretty solid, but a few more wouldn't hurt.