There is little question that the poetry of Shakespeare's Romeo and Juliet is absolutely stellar (forgive the pun). The light/dark imagery extends from the opening prologue which mentions the "star-crossed" lovers until the end of the play. Each line is written in iambic pentatmeter, giving a melodic lilt to each passage.
When Romeo first sees Juliet, he is star-struck:
Oh, she doth teach the torches to burn bright!
It seems she hangs upon the cheek of night
Like a rich jewel in an Ethiop's ear--
Beauty too rich for use, for earth too dear!
So shows a snowy dove trooping with crows
As yonder lady o'er her fellows shows.
In this passage, Shakespeare employs simile, alliteration,and light/dark imagery to express Romeo's infatuation with the lovely Juliet. Then, when Romeo does speak to Juliet for the first time, their language is elevated to the form of a sonnet in which each speaks seven lines of the extended metaphor of his lips being pilgrims on their way to the "holy shrine" of Juliet's lips. Romeo falls immediately in love; he adores Juliet, and begs her to grant him a kiss, "lest faith turn to despair." He tells Juliet, "Thus from my lips by thine my sin is purged." [perfect meter!]
The imagery of the sonnet is that of reverent devotion; Romeo and Juliet's love is pure and holy, unlike the infatuation that Romeo has felt for Rosalind. Rosalind now becomes associated with darkness and Juliet with lightness.
My favorite teenage love line in Romeo and Juliet occurs after Juliet scans the room when talking to the Nurse asking who all the men were. When she gets to Romeo, she betrays herself. Girls like to keep their crushes to themselves but often can't help themselves from bearing it all wearing their emotions on their sleeve. Juliet is no different.
In one swoop, Shakespeare reveals Juliet's crush with both a simile and a bit of foreshadowing with the words:
Go ask his name: if he be married.
My grave is like to be my wedding bed.
A moment later, after learning of his identity, Shakespeare uses parallelism and repetition, maybe even a play on words:
My only love sprung from my only hate!
Too early seen unknown, and known too late!
It's interesting to me that the more intense feelings grow throughout the play, the more of we see Shakespeare's craft with language.
I would say that there are two major ways in which Shakespeare uses figurative language to show us what the two feel for each other.
First, Romeo feels that Juliet has really improved his life and his feelings already. When he talks about her, all his words have images of light. She is brightening his outlook on life.
The second thing is that the way they feel about each other is pure; their love is pure. We see this because much of the figurative language in the scene is religious (the talk of saints and holy palmers kissing and such).