FF6 Battle Background Palette Helper

Is there an easy way to just do that in hex or do I gotta go through all this?

Yes the easy way is "all this", meaning editing the palette correct yellows in Gimp and importing it in the ROM, unless you can blindly and magically edit a palette in a hex editor.

Edit: You cannot edit graphics with this tool, it only edit the palette.

Edit2: Sorry if I sounded a bit rude, but you can't tell which colors are yellows and to which hex value to change them by looking at 96 bytes in a hex editor, you need some sort of visual support and conversion tool, which is what this process do.