>>27800213While having premade BGs saves time, you also have to take the mon's base color palette into account, such as replacing white with the next lightest color so that when you convert the image to index w/16 colors, you don't end up loosing a lot of the mon's original colors in the process when it tries to keep both colors within the indexed palette.
You can't just add the mon on a BG and expect it to be 16 colors automatically and work.
Example is pic related; each of these portraits uses different, lighter shades to convey the white light in each mon's eyes. Understanding how color works helps with how you can keep them within the palette and look natural.
