Uh, try this out:
If you are getting error then keep redoing the same thing until you get a Type Mismatch.
Don't press OK YET!
Now you slide aside the prompt window and click the original APE window. Now press Alt + Prnt Sc | Sys Rq. And then open a new Paint folder and press Ctrl + V.
You will see that the original window of APE has some colours in the changed pallete section. Now open an APE WIndow again and load your ROM.
Go to the offset again.
Now see the Paint file and copy the exact codes on it into the changed palette section.
Save it; you're done now.