The first quotation makes use of a classical literary allusion, and makes us think of the dawn is more than a natural phenomenon: It is a goddess who helps ensure the knight's victory.
The second makes use of a simile that evokes several meanings, including the idea that a youthful appearance is fleeting.
I explain and develop these points below.
1. Classical allusion: Spenser's words allude to Aurora, the goddess of the dawn.
This quote is taken from Book I, Canto XI. For background, consider what has been recounted just before these lines. The knight has fallen for a second time, but luckily he has landed in a balm that has flowed from sacred tree. The dragon "durst not approach" because the life-giving force of the balm is anathema to it. So the fight comes to a temporary standstill, and night falls.
The balm is healing the knight's wounds as he sleeps. But Una is still understandably frightened for him. The night is intrinsically worrisome, "noyous," or afflicting. And so she keeps watch, prays, and waits "for joyous day."
The next lines are the lines in question. Spenser tells us that dawn arrives in a poetic fashion -- he alludes to the classical goddess Aurora. She brings light by awakening, rising from her dewy bed, climbing into her chariot, and riding across the sky to
"chase the chearlesse darke / with merry note her loud salutes the mounting larke."
Note that Spenser could have chosen to tell us about the arrival of daylight in purely naturalistic terms. That would have contributed to the feeling that the knight and Una face the dragon alone. But instead he's conjured up the image of a pleasant goddess who intervenes and chases aware the gloom "with merry note." The effect is to make us feel that Una and the knight aren't alone. There are other, supernatural beings on their side, beings that represent good cheer.
2. Simile: Marvell's narrator is comparing a woman's youth to a fleeting characteristic of the morning.
This quotation comes at the beginning of the last stanza, and we can appreciate Marvell's use of the simile better if we consider what leads up to it.
The narrator begins the poem by considering what life would be like if he didn't have to worry about the passage of time. He could spend hundreds of years praising his mistress's beauty.
But in the second stanza, he notes the inexorable reality. Though his mistress might deserve this timeless state of affairs, it isn't going to happen. Her beauty will fade, and they'll both die. Her "quaint honor" -- her resistance to go to bed with him -- will "turn to dust," and his lust will turn to ashes.
He's expressing the age old argument for putting propriety aside and indulging in a sexual affair: Time is running out. We should grab happiness while we can. What will our concerns matter after we're dead and gone?
So when Marvell finally presents us with the quote about youth and dew, he's chosen a simile that emphasizes the fleeting nature of youth. Furthermore, his choice suggests that youth is a mere appearance (a "hue"), something superficial that sits on the skin. Dew appears on a leaf purely as a function of the time of day: We can't take credit for it, nor can we can't change its inevitable disappearance. The dew, the youthful veneer, fades and we are left with the slow decay of what's underneath. Marvell's simile suggests the image that his mistress is in the morning of her life, and that the passing of time will soon dry up her youthful appearance -- an image that echoes his earlier references to the lovers' qualities being reduced to "dust" and "ashes."
Some might think that the reference to a "youthful hue" is itself a kind of metaphor, because youth is a physical state rather than a color. But the English of this era had a highly developed set of cultural beliefs about various shades of skin, and often characterized youthful skin as rosy. So I don't think we can be sure that Marvell is being entirely metaphorical.