I've been researching this for a week and still have not proceeded the source. However, I found the palette set located at ROM OFFSET x10D976 if you use the headered SNES rom, when I edited the hex value, it won't change the palette set for each overworld level. Then I started seeking RAM or a Pointer location to see where it came from. Which is found in ROM OFFSET x14C0AA started at the LDA location BD XX XX followed by the STA location 8D XX XX where the palette code came from.
I don't understand why BD XX XX in x14C0AA would point to a x10D976 and does not make sense. Even if I try finding where the unused slots in the x14XXXX location, it's still not functioning unless I have to find the bankswitch or something.
Now I've been forced to create a custom palette for each overworld level and it's going to take a whole lot a space plus hardcoding stuff which I have 0% knowledge of and I don't know how custom palette works when I'm hardcoding 65536 ASM.
I'm still trying to seek the main palette sets for each overworlds.
I've provided ROM OFFSETS for World 8 Overworld palette and I used vSNES by savestate to find them in the ROM.
x1E2280 - World 8 Palette 1
x1E22A0 - World 8 Palette 2
x1E22C0 - World 8 Palette 3