For Portrait, it is difficult because you have to 16 colors,
but for backgrounds, you can use AI-created backgrounds in games with a little ingenuity.
This is because BG has 8 palettes available.
I use FEBuilderGBA 6 palettes for color reduction, and the remaining 2 palettes are used to manually process the areas of concern individually.
However, if the image has gradients, the difficulty level increases.