In Romeo and Juliet, what is Romeo's mood? How does this contrast to the opening scene of the play?

Romeo's mood would be difficult to pin down.  As Romeo is given to very strong emotions (and can, likewise, be described as both emotional and passionate), his mood vacillates wildly throughout the play.  At times, he is brooding and incredibly low.  His mood prior to the Capulet party, for example, when he longs to see Rosaline, is sullen and sad.  On the other hand, on the occasion of meeting Juliet, Romeo soars to new emotional heights and a near-euphoric mood.  He plunges headlong into despair once more after he kills Tybalt and is subsequently banished from Verona.  And on it goes -- from peak to valley with little emotion in between -- throughout the play.

One contrast we can make with the opening scene, however, is that Romeo is not necessarily violent.  While the opening scene is full of fighting, Romeo is more of a lover than a fighter.  While he does ultimately kill Tybalt, he is motivated only by revenge and by a sense of honor to avenge Mercutio's death.  One does not sense that Romeo would have otherwise committed murder.