Giga Bowser's palette is found:
x1FF682-x1FF69E(0x2 bytes) - Giga Bowser's palette
The results are shown below:
One problem. When Giga Bowser gets too close to Yoshi, its palette almost goes back to normal:
EDIT: wait a minute: I found the problem! Hold on for a few minutes.
EDIT2: Giga Bowser seems to use a second palette when he gets too close to Yoshi. Found the address with that palette: x1FF6FA-x1FF716(0x2 bytes). Here is the final result:
Bonus: Both rising and falling rocks' palettes have been found. The rising rocks palette is at x1FF6A0-x1FF6BC(0x2 bytes). Don't change the pink colors. If you change the other colors, you can get this kind of thing:
And the falling rocks' palette is at: x1FF6DC-x1FF6F8(0x2 bytes) again don't change the pink colors. Should you change the falling rocks colors, you obtain this: