Their respective yellow and light green colours are on pair as well, not being too different from each other. Both have roundish faces, with Piper's head being completely round. Other than that they don't really resemble each other that much, since Piper is very slender whereas Shrek is overweight.
I have shipped Piper Pinwheeler and Shrek together - as friends, not romances - ever since I first laid my eyes on Piper. Ever since before I found out that Piper was female - though I guessed she was female before my guess was confirmed correct.
In spite of their physical similiarities, they seem to be opposites in some aspects of their personalities. Shrek is an ogre who doesn't care about his stink, if anything he's proud of it. While we don't get to see much of Piper's personality regarding cleanliness or dirtiness, I'm sure most of us can safely assume that she would be disgusted by Shrek's habits.