Actually we need to first get the pixels and obtain the RGB component values and from there calculate their percentage from all white and each other then from there should be able to determine which is the dominant/accent color. RGB is easy to extract because they're all contained in 8-bits or 1 byte.

Usually it's stored in 32-bit for alignment/performance issues so the last byte (the alpha channel) is usually not used in background wallpaper, only used as textures in 3d graphics involving the GPU.

For certain formats, the old style 16-bit, you have to determine whether it's in 565 or 1555 but I think that's quite old already, and now most systems should use 888 in 4 bytes.