First, you have to find the palette. Using a debugging emulator like RegenD, GensKMod, or Exodus, load up the rom and play until the palette you wanna change is displayed. Load up the VDP debugging window, dump the CRAM (which holds the palette data of the 4 palettes), select the palette you wanna change, then use a hex editor to see if you can find it in the ROM.
But as with most things, it's not always that easy!
What palette are you trying to change, I could have a little look when I get back to my real computer.