Light and dark imagery can be used only to show contrast, helping to highlight one thing by showing the opposite. For example, in the balcony scene, Romeo describes Juliet as the sun:
It is the east, and Juliet is the sun!--
Arise, fair sun, and kill the envious moon,
Who is already sick and pale with grief,
That thou her maid art far more fair than she:
He says that Juliet is so beautiful, so "lit up with beauty" that she will kill "the moon" or, in other words, the night. Her beauty will make it daytime. These contrasts are just to show how extreme Romeo's admiration of Juliet is.
Traditionally, light has symbolized good and dark evil, or at least not-so-good. While this is sometimes the case with Romeo and Juliet, the standard interpretation is not always true. In the traditional sense, the balcony scene fits the criteria, for Juliet hopes for the sun to banish the "envious moon" and turn night into day.
But on the other hand, when the lovers spend their first full night together as man and wife, day is shunned for the pleasures that night has delivered. Both the young lovers try to try to pretend that it is still night, and that the light is actually darkness. Romeo hears the call of the lark, a bird of the morn, but Juliet desperately wants to pretend it is a nightingale they've heard (thus meaning it is still evening):
She says: Wilt thou be gone? it is not yet near day: / It was the nightingale, and not the lark, / That pierced the fearful hollow of thine ear; / Nightly she sings on yon pomegranate-tree: / Believe me, love, it was the nightingale.
And Romeo replies: It was the lark, the herald of the morn, / No nightingale. Look, love, what envious streaks / Do lace the severing clouds in yonder east (3.5.1-9)