I’ve recently started replaying Final Fantasy Tactics Advance, and noticed something I hadn’t payed attention to previously: it has 3 color modes, LCD A (which I reckon is the one to use if playing on an original GBA), LCD B (which I reckon is what you should choose if playing on some flavor of GBA SP) and TV (which is for when you’re playing on a Gameboy Player, I guess). The alteration to the color palette is pretty nice, and seems more “natural” to my eyes than something like nds-color or gba-color, so I wanted to replicate whichever changes the game makes in a shader. My idea was to somehow compare this:
To this:
And replicate whichever transformations the game makes to the palette to turn A into B. The problem? I know next to nothing regarding shaders, and just a bit more regarding image manipulation.
I’ve read a bit, and, as informed by hunterk, image-adjustment and color-mangler should be able to do anything I need with colors, but, save for diving into the game’s code and seeing how it does it, something which I have no clue how to do, I don’t think I’d be able to faithfully recreate the look that easily. I decided to try my hand at another of hunterk’s advice in previous threads, and make a tone-mapping LUT shader. However, I don’t know how to automatically compare the two images, see how they are different from one another and replicate that difference on the color swatches. I can spot the differences pretty easily in Photoshop by layering one image over the other and changing the blending mode to difference or subtract, but that’s the extent of my knowledge on the subject. Messing with colors, luminosity and levels could probably take me very close to that, but that’s way too much work for a task that, all things considered, could probably be easily automated.
So, I’m kind of lost and without any ideas 
Can anyone point towards material I could read to learn basic stuff related to tone-mapping, or help me achieve what I want? Thanks in advance, everyone!