If you’re looking for a different way to draw in PictoChat, consider coloring in your entire screen so that it’s black. Then, use your eraser tool to draw on the black, effectively reversing your color options!
There are currently six versions of DS firmware. To find out which one you have, have a game in your DS and enter Pictochat Room A. Remove your DS game, and the screen will briefly turn into a specific color. The color corresponds to the version of DS firmware you have on your DS.