Colors
Metals
Gems
something, gems?
This is Colors
however, I'm taking a guess on this that
Black will be the darker one
White will be the lighter one.
It would need to be something that goes inbetween this logic.
Pokemon Grey is out of the question because of Silver being there. Silver works as a color too, so this would cause confusion with Soul Silver.
Pokemon =
Nuetral, Chrome, Fate, Twilight, Dawn, and Legends.
These few are some of the possiblitys that work. I have speculations on Chrome, as it would work rather nice.
Dawn and Twilight are questionable as Dawn was already a character in the series, and twilight gets thought up now a days as sparkling vampires.
(I'm thinking of the Kingdom Hearts/Paper Mario name of there Twilight Towns, and KHs Twilight "power". If you get this part, then you know why I say so)