I'd like to use BizHawk to do some palette swapping on NES ROMs. I'm guessing the emulator's built-in Hex Editor can do this task, but I didn't have much luck trying to get the job done.
For example, let's use RockMan 2 as test:
I opened the PPU Viewer with the rom, then opened the Hex Editor, then went to "Options>Memory Domains>PALRAM. Let's say I want to change Air Man's stage background from blue to red. To do that, I know I need to edit the first palette address (which is "21" on this case) to "05". Basically, address 21 is a shade of blue on the NES palette chart, and address 05 is a shade of red.
This is how the screen would look like (edited through Photoshop):
But how can I actually edit the 21 into 05? I tried poking the address but the emulator won't let me change it. What should I do?