On top of this, Nintendo of Japan is releasing some limited-edition Mario DSes with wireframe art of Bowser, Peach, Mario, Yoshi and Wario on them (and a different colour for each character).
My guess is that NOA is shying away from bringing lots of new colours to the market in the U.S. for fear that it will make them look immature. Look at all the bright N64 controller colours we had, or all the GBC colours. Then everyone freaked out when they used Indigo for the main colour of the GameCube. I'm also guessing new colours don't have as big of an impact on sales in North America. I could be wrong about that one, but it seems like most people here just want black or silver, or maybe white.