You've probably heard this saying about a billion and one times, but since it's so very correct, we think it bears repeating: "Don't judge a book by its cover." Really, don't do it. As we see in The Picture of Dorian Gray, you can never be sure what's lying beneath that fancy-pants, super-exciting cover – it may look gorgeous on the outside, but it could be totally poisonous on the inside. The protagonist of this novel, Dorian Gray himself, is a case in point: he's unbelievably beautiful on the surface, but his soul is grosser than gross. However, throughout the novel, he gets away with the most dastardly things, simply because he looks too innocent to do anything wrong.
The flaw in the Decadent philosophy suggested by Lord Henry and the yellow book is the idea that there is nothing beyond surface appearance.
The confusion of appearance with morality is what causes the downfall of every character that interacts with Dorian.