...except that Diamond and Pearl are precious materials far more than they are noted colors. "Ruby," "sapphire" and "emerald" can at least normally be classifed as basic primary colors with some manner of gleam or shine to them; the production lines accomplished this with translucent plastic. Diamonds, technically, are best known as colorless, but Game Freak elected to associate diamond and Dialga with a shade of blue. Likewise for pearls: normally an off-white, with most of their perceived color coming from reflected light, but Game Freak chose to link pearl and Palkia to pink.
Regardless, not only is there so relatively little exposed plastic on a DS game card once the label is applied, but DS cards disappear entirely inside of a DS, where Game Boy cartridges were either partially or completely visible on just about any Game Boy model but the Advance SP. So it all boiled down to Nintendo/Game Freak not finding it worthwhile to press the game cards with colored plastic because there would have been no novelty to it and no publically-visible identification with it unless you actually held the game card in your fingers and slowly turned it about while ooh-ing and aah-ing.
And the best part is that had Nintendo elected to put forth the extra production costs necessary to manufacture media from other-colored plastics, you would have had people complaining about having to own a pink game card. Proof that the masses will never be truly pleased with anything.