What if Mudkip adapted to polluted waters instead of clean waters? Well it obviously turns into a poison type. So yeah. The reason it has so many colors/shades it is because poisonous frogs are colorful to warn potential predators that they are poisonous. I know Mudkip isnt a frog but it and Marhstomp look amphibian and thus it fits. I also changed/added slightly different things depending on the evolution.
Mudkip has nothing as that is the base.Marshtomp tail and head crest have different colors than before.And now it had added a toxic symbol on its chest/stomach area.Also the face shading to say so it is vertical now instead of orizontal.And its chest/stomach is purple.It also has a poison symbol on that area.
Swampert face is now zigzag like a chess board alternating between light and dark purple.It also has indigo/dark blue combined with the color from the poison symbol teal/dark green for a bit of extra colors. And of course its chest/belly it is completely dark blue like it was when it was a Mudkip.So that area goes from blue to purple and then to dark blue.Similar to tail and head crest colors.It goes from indigo/dark blue with dark purple to blue with red and finally dark purple with red.Just to combine and shift them around a bit. You can see all these differences if you zoom out the sprites a lot.