You can’t stick a Game Boy Color cartridge in a Game Boy. Most of ‘em won’t work. Here are almost 1000 examples of games telling you that.
The Pokémon Trading Card Game for the Game Boy Color is making its way to the 3DS eShop... in Europe. The game will arrive on Thursday, and will cost €5. We've contacted Nintendo to ask when (or if) the game will come to the US eShop, and will update you should they reply. (via Kotaku UK, image from GameFAQs)