Upcoming Versions
The versions available in Japan are known to all, which are Silver, Gold, Yellow and Green.
Silver and Gold Version:
Silver and Gold versions will have a new look to things. They will
still be available on Color Game Boy with other new features. From what I heard, there will be more than 250 pokémon in this game and all have their own gender. Not getting confused if Pikachu was a girl or a boy, and for some people who don't know Pikachu IS a girl.. In the cartoon of course, but you wouldn't be so sure because they didn't specify it is so. Of course, if you have gender of female and male, they would be able to make other Pokémon by laying eggs. The funny part is that you could use different species to different species. Then you'll see what comes up!
The game with have it's own clock so it will not be daylight all the time. Also, they have added many new features to the Pokédex. First, you could know a Pokémons next evolution! As you already know there will be more than 250 Pokémon, which means that the old Pokémon have new evolution forms, but they also include the old ones. Next, your will be able to use a skateboard instead of a bike, as you used in the blue and red versions.
Remember the game link cable?? You'll be able to send Pokémon over to the new version! They will have new information and data on the Pokémon. But you'll not be able to use the Pokémon you have in your other(other than gold/silver) version you sent to the new version.
Yellow Version
The Yellow version is based on the popular cartoon. Where the yellow came from Pikachu. Pikachu is yellow, Pikachu is the main character so it's Pokémon Yellow. There is 151, yes 151 pokémon in the game!!
The main difference between the yellow version and the rest of the others, Pikachu will follow you around just like the cartoon. Most of the plot in the game is similar to the cartoon. And if the plot is similar to the cartoon, everyone would be there, yeah, Gary (who actually looks like Gary) and even Team Rocket and other various characters like Nurse Joy and Chansey!
A friend of mine currently has the Yellow Version and the strategy guide, but all in japanese though. Anyway, I flipped through the pages of the strategy guide and the pictures were much better made than the ones in the Blue and Red version.
Rapidash, with your top speed bring the visitor back to the main entrance!