Extracting and colorizing Pokewalker Sprites!
The Pokewalker is a handheld pedometer that came bundled with HGSS, and it has a bunch of two-frame grayscale sprites. The sprites used on the walker are taken from Diamond/Pearl and positioned so that they'll fit on the small display, but lack any color information.
Below are the original grayscale Pokewalker sprites (with transparency), and their now fully colorized counterparts. Drag the slider to see the difference, or click one of these direct links: Grayscale - Color (Slider code from this post):
In 2020, Dmitry.GR posted some information on reverse engineering the protocol, as well as the sprite format used by the device. These sprites are not actually stored on the walker though! HG/SS transfer them over to the Pokewalker each time that you sync with it. These sprites can be extracted from the HG/SS DS ROM.
Then, the original full-color Diamond/Pearl sprites can be exgtracted and aligned with these grayscale counterparts, to create an accurate representation of what the images would have looked like if they were colored from the start.
Some code used in this effort is here: https://github.com/vgmoose/pypokescript
I didn't yet go through all the alternate forms, but that might be a goal in the future. As for why do this with the sprites, maybe they could be useful in creating full-color recreation of the pokewalker. Or adding a color overlay to a Pokewalker emulator!