I don't think going off their colors will do much good, if that's how you're inferring the types of the legendaries. For example, who would've guessed, at first glance, that Reshiram was part FIRE? His color is white, with blue eyes... Nothing to suggest fire in his appearance. And many people thought Zekrom would be part Dark, because he's all black, but he turned out to be part Electric. Like I said -- you can't necessarily tell from the colors.
Perhaps they will instead by focusing on creating type combinations that haven't been done before. In which case, maybe Yveltal is Fighting/Dragon (fitting, if he's based on the Norse myth of the bird atop Yggdrasil), while Xerneas might be Rock/Dragon -- if that seems unintuitive, look at Terrakion's design and see how the creature needn't actually look rocky. And then if the third legendary were Poison/Dragon... A nicely balanced trio, no?
But it's all speculation at this point.