The Prologue is the first place that I'd start because of Fate's voice right at the beginning that warns the audience of what will happen in the play. "A pair of star-crossed lovers take their life,/ Whose misadventured piteous overthrows/ Doth with their death bury their parents' strife" (Prologue. 6-8)/ Fate, it can be argued, is the paradox; that no matter what any one character does to mend or fix things, the lovers will still die. This paradox is one of the most famous and greatest.
The next quote, one of many that would do for your essay, is right before Juliet discovers that Romeo is a Montegue; "If he be married,/ My grave is like to be my wedding bed" (I.v.132-133). Not only is this a foreshadowing of what is to come, but it is a simile that brings about a sad and dark image. Along those lines, Juliet continues to speak after she finds out that Romeo is a Montegue with, "My only love sprung from my only hate!? Too early seen unknown, and known too late!" which employs the that shows the conflict within as well as without (I.v.135-136).
After the Prince hears all of the accounts of the story in the end, he uses an oxymoron to show the pull that good and bad has had on the whole story by saying, "A glooming peace this morning with it brings./ The sun for sorow will not show his head" (V.ii.304-305). Morning is a time of awakening with hope and glory; sadly, that's not what happens on this morning. the images of light and dark are found throughout the play and can be applied to the theme of forbidden love even at the end as it is shrouded by darkness.