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You just need a hex editor, I've done this myself and I'd write a tutorial if it weren't for the fact that some of the palettes I've added glitch and I've yet to find a solution as to why.
Check this earlier thread of mine and you'll know what I mean by glitch...
yeah, you just have to reposition the ow/palette table to a free space and repoint any pointers to the new location of the tables. Then add new ow/palettes and entries to the tables and have them point to the new ow/palettes.
Check this image out if you please :)
For some reason even though I have pointed the the camper's ow to the empty slot D, it doesnt add it's palette to the palette slot, what's more brock's ow fills up two palette slots for some reason. '.'
I've added four new palettes to firered through an hex editor, moved the palette table to a new location and repointed everything to it. Each palette was assigned to a different OW, three of them seem to be working fine but the last palette I've added is glitching. I've checked it's overworld...