Matthew's Jesus alludes to the common Jewish practice of whitewashing tombs to set them apart from their surroundings so that people would not unwillingly walk among or upon them, thereby unknowingly becoming corpse impure. He denounces his opponents for not being what they seem. They appear pure and righteous on the outside--white as snow-- yet they are actually full of impurities due to their lawlessness --that is their moral impurity (Matt. 23:28).  He does not criticize the Pharisees for being punctilious observers of the law, or "legalists" as Protestant theology often calls them; rather, he censures their hypocrisy and their lack of lawful, righteous living. The Pharisees simply are not righteous enough according to Jesus. -Matthew Thiessen, Jesus and the Forces of Death, p111