The OS isn't using a palette to draw to the screen. The LCD is in the 565 RGB color mode, which does not use palettization. The actual color values themselves are stored inside the OS, which can't be modified without the calc becoming inoperable after a reset.
You can affect the LCD controllers' parameters to a certain degree, but most of the things that you can modify are either completely unrelated to color or would prevent the OS from displaying anything intelligible. You can invert the colors, however - there should be code floating around for that; if not, I can either find it or rewrite it for you.
Your best bet might be to use an OS hook, which is a TI-OS feature that allows you to intercept when certain events in the OS occur and run your own code. You could use the localization hook to draw your own text, which you could make any color, and then use another hook (probably either homescreen or cursor - you'd have to look through the
list) to replace all the white pixels on the screen with blue each time it scrolls. There's not really a user-friendly way to use hooks at this point, however - if one program uses a hook, it prevents all other programs from using that hook. I'm currently working on a
program that will hopefully fix that issue, as well as making it simpler for people to install hooks. You'll still have to write hooks in assembly, however - the linker used in the C toolchain doesn't support them.