Ok, well, I have a lot of ideas and theories going on right now. (Well yesterday I did) I wrote it all down in a notebook in bed (lol) here it is:
I think it has an equal chance of being Grey and another color. First I'm going to review some of the other generations. (Not counting remakes, unless they are a new color or something!)
Red, Blue, Yellow- Primary colors, of course!
Gold, Silver, Crystal- There's only one thing I can think of, that they're all rare-ish and can sell for a lot? Gold and silver have been used for currency, but I'm kinda not sure on the Crystal.
Ruby, Sapphire, Emerald- They, I guess, are kind of opposites in the gem world. Or something. As for Emerald, well they already had red and blue, so it couldn't be a yellow one, so I guess it was green! Plus these are all gems.
Diamond, Pearl, Platinum- I can understand those, both gems are kind of palish with a hint of another color (blue, pink) in them. Platinum, I guess, it was another yellow version in a way. I am just confused as Diamonds and Pearls... Then a metal? O.o Ah, but, Blue, Pink, Yellow? Sound familiar? ish?
Black and White- Yin and Yang, since Reshiram is on the Pokemon black cover, and vice versa.
Ok, now, let's review: 3 yellowish games, Yellow, Gold, Platinum. Only other idea I could think of is maybe Topaz. Huh, actually a pretty good idea.. Wait, Topaz is yellow, right? 3 bluish ones, Blue, Sapphire, and Diamond. Silver and Crystal are both white/clear ones, I guess, and if you count Pokemon White, that makes 3 white-ish/clear ones. (Unless you think of crystal version as a blue one, Suicine is slightly misleading) Now, Emerald and LeafGreen(the remake?o.o) that equals 2 green games. But what's this? Only 2 reddish games? (ruby and red) We're missing one! And Red is an.. this sounds stupid... important color in the pokemon generations... actually that makes no sense... Never mind. So another red one is possible. Unless you count Pearl into the mix...
Red.................................3
Blue................................3
Yellow............................3
Clear/White...................3
Green.............................2 (!!!)
So it's also possible it could be green! And it's not gonna be black or whatever because that would make it Pokemon White, Black, and another Black O.o
Anyway yeah, that's what I think. And I'm not saying my prediction is 100% correct or anything, it's just my theory.
P.S. If it were Kyurem it could be grey because Kyurem is part grey! And if it's not then it might be light blue but I don't think that fits very well into Pokemon Black and White (and light blue XD)
P.P.S. I know you might ask 'If it was green what would the mascot be?' Well I don't know I was simply adding up all the different versions and saw that green was behind. (so is black but whatever)
P.P.P.S. But if it were red as some of you have brought up, what would the mascot be? Reshiram is already part fire! So he's like white/red. Zekrom's part electric and his electricity's blue, so, that leaves other stuff.