Frankly, I am a little surprised we haven't seen those colors already.  Let's hope they don't go too Spider-man 3 on us. 

We don't know much, except that we will hear more on April 15th, but they decided to release the logos (above) before any real details about the game on Pokemon Sunday.  They also have put up the teaser site.

However, Junichi Masuda has posted an announcement on the GameFreak website.

It's in Japanese, so after running it through a few translators and trying to clean it up with perhaps too much inference (put it in Google Translate yourself for literal), this is what I've got:
Quote
We can finally reveal two completely new projects we've been working on!

"Pokémon Black"
"Pokémon White"

In order to enhance the quality even incrementally,
we have to put a lot of work into it.

We are working to make it so that veteran players can enjoy it.
We are also trying to make it so that players new to the Pokemon series can enjoy it.
And this time, people think that Pokemon graduate. (My guess interpretation: Even people who thought they had seen it all with Pokemon will enjoy it.)

We want many people to enjoy this game.
We produced these games with that in mind.

These versions will revolutionize the Pokemon series.

Please stay tuned.

Ciao!