In the world of fairy tales, beauty and goodness are almost always synonymous-physical beauty is usually representative of inner character. The witch is intriguing because she is evil and yet beautiful. She is not only murderous, but cannibalistic, wanting to eat Snow White's heart and liver (perhaps to somehow absorb her youth and beauty through the act).
Snow White herself stands out because of her black hair. When a beautiful fairy tale heroine's hair is described, it is almost always blonde-and not just "blonde" but "golden," so we are reminded of more than fair hair, but also of wealth and royal status. Thus "Snow White embodies a peculiar dichotomy; whereas her stepmother is bad but beautiful, Snow White is good, but raven-haired." So the characters' very appearances, which are integral to the plot, stand out against other fairy tales. (EDIT: Julia Mavroidi mentioned in the comments that Snow White's hair was not always black-in fact, in the first edition of the Grimms, it was her eyes that were ebony!)
The apple is the most iconic image of Snow White, but the apple only comes after two other temptations-stay laces and a poisoned comb. Other people have analyzed these objects as representing Snow White's vanity, an observation that chillingly likens her to the witch. And while there are other reasons to suppose that Snow White is potentially in danger of placing too much value on her own beauty, maybe that assessment of the temptation objects is a little unfair.
First of all, in my opinion, we have to remember that the disguised witch offered the goods to Snow White. There's a difference between seeking things out and accepting what is offered-she is not like the sisters in Beauty and the Beast who seek out dresses and jewels.
Dolce and Gabbana Fall 2014
Fairy tale inspired line
But Colleen Hill also reminds us that Snow White was a princess. She was used to having a vast wardrobe of the latest fashions, and also probably someone to dress her. Even though the story doesn't indicate she is discontent with her new life with the dwarfs, it's safe to assume she might miss some of her old pleasures from the castle. In fact, the witch begins by a not-so-subtle insult; "Oh, my child, what a sight you are. Come, let me lace you up properly." In addition to that immediate pressure to improve on her looks, Hill says, corsetry "had other connotations as well, such as beauty, social rank, and morality." We know that Snow White values neatness and cleanliness in her new duties as housekeeper for the dwarfs, so it's natural that the desire to have a tidy home would also extend to a desire for a well kept appearance.
Probably the thing that most hints towards Snow White's vanity is the fact that the apple is described as being appealing to look at-"white with red cheeks"-just like Snow White. Yet those who condemn Snow White for desiring a pretty apple don't condemn the Prince for desiring Snow White for that same feature (while dead/in a coma)-although in the Grimms' version he really should be.
Hill also points out how much pressure was put on royal females to appear beautiful. Queens would be sharply critiqued for perceived flaws in appearance, yet paradoxically also condemned for being too vain. You could say that the Queen witch was really a product of her culture, just as young Snow White's desires for pretty things came not only naturally but through passed down values. In fact, this pressure contributed towards historical figures who were more or less real life evil Queens-Catherine de Medici supposedly tried to eliminate Louis de Conde with a poisoned apple, and St. Ludmila (the grandmother of King Wenceslaus) was strangled with a veil by her mother in law.
The pressure to look beautiful is not an archaic relic from a misogynist past, but alive and well today, as women are continually pressured by media to find value in their looks, and told that men will only love them if they adhere to certain standards. Which is why Snow White is still such a powerful tale, and just as it indicates that the desire for beauty is passed down through generations, so it is hundreds of years later...