Possibly to set him more apart from the other brownish character whose head was his favorite resting place? Just guessing there, but perhaps some illustrators thought that each of the five characters should have a very distinctive color pattern.
What I consider more "troubling" is the "bat version" of Petrie which can be found on so many products, a version of Petrie that looks nothing like the movie Petrie. A bat-winged creature with some hair on the head as well as the throat, an extremely crooked beak, and very thick rings around the eyes... that thing looks ancient rather than like a young flyer. I wonder who ever came up with that one and for what reason.
Anyway, there is plenty of Petrie stuff out there ranking from various movable and unmovable plastic figures (including a waterpistol) in different sizes to plushs (many different version there) to his depiction in books (often along with other characters) and on various merchandising. He seems to have been the most challenging of the gang to depict. At least I know of at least one series of merchandising products (spoons and cups) in which he was the one of the Gang not represented. In games to he seems to be more prone to be made some kind of adviser (in the stile of navi in Ocarina of Time) rather than a playable character.