"Look, love, what envious streaks. Do lace the severing clouds in yonder east"(3.5.7-8)
"Yond light is not daylight, I know it, I. It is some meteor that the sun[exhaled]. To be thee this night a torchbearer"(3.5.12-14)
Romeo and JulietJuliet's room
"Let me be ta'en; let me be put to death. I am content, so thou wilt have it so. I'll say yon gray is not the morning's eye."(3.5.17-19)
Romeo came from Mantua and sneaked into the Capulet mansion to spend the night with Juliet.
O, now begone. More light and light is grows."(3.5.35)
"More light and light, more dark and dark our woes."(3.5.36)
The motif of light and darkness is evident in their dialogue as they use light as a means to express their ideas. Juliet wants Romeo to stay by saying the daylight is rather a meteor that will light his way to Mantua, "Yond light is not daylight, I know it, I. It is some meteor that the sun[exhaled]".
The idea that Romeo is willing to die just to be with Juliet for one night shows his sincerity to be with her. He would rather pretend that the daylight is not "morning's eye" but rather "Cynthia's brow" which is the reflection of the moon. The moon could be the darkness that doesn't allow Romeo's love.
Shakespeare is expressing, through Romeo, sorrow for both Romeo and Juliet as the their love blossoms in the night when no one is there, but ends in the daytime where they have "dark woes". You could see it as a contrast as their love which could be seen as light only shines in the dark. But the light disappears when the day strikes and Romeo and Juliet's dark thoughts set in.