I feel your pain. Anything not 1bpp confuses the hell out of me in FEIDIAN. I want to try and code a new version that's easy to use, but first I need to understand how the old one works.
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You know what you may want to try to help learn? Open up Photoshop and recreate bitplane graphics. The concept is easy. You've got multiple simple 1bpp layers stacked on top of each other.
You want to learn the different formats, right? So how about this. Draw the 1bpp layers out in Photoshop for any example you're working with. Now, you can actually stack them on top of each other yourself and see exactly how the final image is generated. Hell, you can even see how the colors work if they are somewhat transparent and you set the palette up.
You can manipulate and size these layers however you need to to recreate what's happening with any format and size.