Pokémon Black and White versions

Even in a white DS, having a white cartridge stands out from other games. I had a Yellow Gameboy Color, but Pokemon Yellow version was different colored from every other game (aside from Donkey Kong Country) so still stood out. Meanwhile, if you had a black cartridge, it wouldn't stand out as much because it's almost the same color as the other DS games. Pokemon silver had this "sparkle" to it, and was a shade that really did look different from other Gameboy games.

I agree, even the Gameboy Advanced had coloured cartridges. Such as Ruby, Sapphire, Emerald, FireRed, and LeafGreen. Making coloured cartridges for the Nintendo DS would be rather pointless for these Black and White versions, but if it was for HeartGold and SoulSilver then a nice shade of Gold or Silver would have been at least accepted.