Technically Black and Black2 are the first in the DS series to have matching color cartridges.
Even though the cartridges are actually kinda of a translucent brown if you look at it through a light.
It's actually a more reddish colour, but perceptively it can look brown under certain light.
It has to do with the infared (IR) receiver in the game card. It's that particular sort of near-opaque translucent black plastic that is needed for an IR signal to pass most effectively to the receiver.
Edit: Because the 3DS has it's own IR port, Pokemon X and Y will most likely not be using that plastic. --- We shall rise again to start anew. White FC: 0862-2660-4156 / 3DS FC: 2492-4470-1418 / White 2 FC: 4556-7836-2644