Short answer: In order to illustrate the outward appearance of beauty, Bassanio gives these examples: (1) facial make-up, (2) wigs, and (3) a beautiful scarf that wraps a woman's head and partially disguises her face and its dark intentions.
One of the longest scenes in The Merchant of Venice, Scene 2 of Act III is also one of the most important because it continues the story lines already introduced as well as setting up the ones to come. There are several dichotomies in this scene as Portia makes efforts to delay Bassanio in order to enjoy his company before he makes his choice of the caskets, but he insists upon making his selection. So, as Bassanio deliberates, Portia orders a song sung, that is used to subliminally suggest the leaden casket as every line in the first three stanza ends with a word that rhyme with lead.
While this song is being sung, Bassanio comments to himself on the deception of appearances with certain examples:
1. He observes the illusions of facial beauty:
Look on beauty,
And you shall see 'tis purchased by the weight,
Which therein works a miracle in nature,
Making them lightest that wear most of it. (3.2.88-91)
At the time, fair damsels were considered more beautiful than the darker, so women often used make-up to make them seem lighter in order to allure men. "...tis purchased by the weight" suggests that the gold casket in its beauty may be equally deceptive. There is also a pun on the word "lightest" in the sense of unchaste, implying that women who wear make-up are respected the least.
2. Beautiful hair, too, can be illusionary
So are those crisped [curled] snaky golden locks,
Which maketh such wanton [playful] gambols with the wind
Upon supposed fairness, often known
To be the dowry [gift of property] of a second head,
The skull that bred them in the sepulcher. (3.2.92-96)
The hair that some women wear may not even be their own; instead, wigs may have come from others now dead as hair seems to grow longer after death. This mention of the sepulcher also suggests a deadly deception in the sense of leading the man to unhappiness and a moribund existence.
3. Therefore, superficial beauty is often treacherous
Thus ornament is but the guiled [full of guile, treacherous] shore
To a most dangerous sea, the beauteous scarf
Veiling an Indian beauty [Eastern Indian, hence dark and dusky]....(3.2.97-99)
Decoration and superficial beauty can often disguise faults and deceive, hiding the darkness of the woman's soul and her real motives and intentions, thus leading a man into "a most dangerous sea" of a miserable life. The superficial beauty may really cover a dark soul; so, like the sailor who leaves the shore where calm waters lie, he may later encounter stormy seas of interpersonal conflict. The woman who seems lovely in every way may not really be beautiful or kind.