Game Boy Pocket was just a Game Boy Slim or Game Boy Lite. Game Boy Color actually had different games for it, but for some reason everybody counts them as as Game Boys.

same as DS.

I am also confused about that. I personally find them to be two different handhelds.

As for the DSi, I think it should be counted along with the other DS models.

Game Boy is essentially like the DS Phat.

Game Boy Pocket is basically like DS Lite - better form factor, but all the same functions.

Game Boy Color, finally with new functions, is essentially like the DSi.

 You can play Gameboy Color games on a Gameboy. I know this because I did it back in the day.

No you can't. You can play Game Boy games that are optimized with color palettes on Game Boys (Link's Awakening DX, Wario Land 2, Pokemon, etc).

But you cannot play Game Boy Color games on a Game Boy.

And what's more, Game Boy Color games won't fit in an actual Game Boy. The cartridges are designed so you can't use Game Boy Color titles in a classic Game Boy.

Only certain games could be played on color AND regular gameboy. Link's Awakening DX was a good example of this. However later games came out that were for GBC only, like Crystalis and Dragon Warrior 3.

Personally, I think we should keep DSi seperate to start until we can determine whether there are going to be enough DSi only games to warrant it.

No one should "think" of course it gonna be included with regular sales.

 Pokemon Gold and Silver seemed to work great and had Gameboy Color scrawled along the side of the box.

Edit: Ahh Grey explained it a bit better.

Wario Land 1 is Game Boy only.
Wario Land 2 is Game Boy or Game Boy Color, but enhanced for Game Boy Color.
Wario Land 3 is Game Boy Color only.
Wario Land 4 is Game Boy Advance only.

They made a few games for both because it was the first backwards compatibility crossover period. We don't get GC/Wii or PS2/3 or XB/360 games to smooth out our generational jumps anymore.