Pleaselog inorregisterto add a comment.
I believe its just the transition from Gen III to Gen IV and even to Gen V. There isn’t a real reason, mostly because the graphics were poor, but just look at these Pokemon who also have color difference from generations (I specifically chose Ghosts to match the theme :3).
Hope I helped. I got all the sprites from here. P:
Game Freak are the only ones who can properly answer this question, but I think it’s becuase the Game Boy Advance had graphic limitations, and couldnt properly render Shuppet’s dark purple color.OR, simply, Shuppet’s first design was grey, and in gen 4 Game Freak decided to change it to purple because they didnt like how the grey color looked (the purple is way better looking imo).