nothing wants, but that thy shape, Like his, and color serpentine may show Thy inward fraud

"Nothing wants," tells us that Adam is saying that all Eve is lacking is the shape of the serpent, and the color of the serpent, other than that she could just as may be a serpent to him.  The fact that Eve literally is lacking the shape of a serpent and the color serpentine  makes her not a serpent in that way.  This is untrue, because unlike Satan in the disguise of the serpent Eve ate from the Tree of Knowledge to help Adam and herself.  Yes, she tricked him and it was terribly wrong, but she didn't do it just to get even with God, and bring his ruling to an end, unlike the serpent, who tricked Eve.  His  reasons were to destroy God and his precious creations, and to bring forth the destruction of man.  To the reference of the color serpentine Adam is referring to it a "dull green" that Eve is jealous of Adam's and God's relationship.  She knows that she was created second, and so she is subordinate to Adam.  That the only reason she was created by God was so that Adam wouldn't be lonely in the Garden of Eden, and so both she and Adam could produce children to help God populate his paradise and pass down his faith.  Eve, in addition is very envious of the serpent's powers, so this made her decision that much easier to deceive Adam.  Eve is not the only one, though he won't admit it to himself of the fact that Satan chose Eve over him to help with his devilish crusade against God and the fall of man.  Adam is also envious of Eve that she had the gall to defy God, and to eat from the Tree of Knowledge, even if it was a direct order not to.