So with Gold and Silver comming, I made a trip down the memory lane.
Up until gen 4, all the cartridges where colored. It wasn't the first time we had colored cartridges (I have a yellow GB onkey Kong cratridges), but it wasn't really common either.
It helped the game stands out around the crow, it made them feels specials. RBY was already cool (even TCG was colored Black? And the Pinball Cartridge was amazing as well, with the vibrating effect that made it far bigger.), but Gold, Silver, and especially Crystal (thos dazzling, shining colours !) were really great looking. RSE where also very cool.
Then Gen 4 arrived, and the cartridge became smaller. They still looked different (black instead of grey), but it wasn't the same thing.
And now, starting with Gen 6, they don't have anything special at all anymore.
This have nothing to do with the games quality, obviously (this could actually be quite the opposite, sinne Gen 4 and 5 gave us some of the best Pokémon games, and Gen 6 and 7 are quite cool as well.), and we now have things like cool Steelbooks or Consoles for the collectionners, which could be considered better deals. (the HGSS Podometer was also amazng).
But this was part of what made Pokémon unique, so I was a little sad to see it disappear.