In rereading Martin Luther King, Jr.'s "I Have a Dream" speech, there are words and phrases that jump out at me, again...and again.
In terms of Rev. King's tone, I think there was a sense of hope, but also the recollection of the dangerous and arduous journey people of color had made to arrive at that place and time, so long after they were promised freedom with the Emancipation Proclamation.
Words and/or phrases that set a "hopeful" tone include:
momentous; beacon of light; joyous daybreak; magnificent words; promise; unalienable rights of life, liberty, and the pursuit of happiness; sacred obligation; great vaults of opportunity; riches of freedom; security of justice; the sunlit path of racial justice; invigorating autumn freedom; high plane of dignity and discipline, etc.
A darker tone is found in word and/or phrases like:
seared in the flames of withering injustice; crippled by the manacles of segregation and the chains of discrimination; languishing; exile; shameful; defaulted; bank of justice is bankrupt; the dark and desolate valley of segregation; fatal; discontent; rude awakening, etc.
In his speech, I believe Rev. King moved between light and dark imagery to recall the hardships of the past, without losing sight of the promise of the future. His harsh words reflected horrific times, while his hopeful words and phrases, by comparison, let people know that the pain of the past was not forgotten, and that it was also not in vain—and that after so many years of struggle and oppression, that a new era was dawning for all people. Rev. King's message was about non-violent demonstration. His words here, meant to move hearts and change minds did not lay blame, but directed all eyes to the realization of dreams long-held, which were only a short way off.
There is no doubt that Rev. King's ability to keep both the dark and the light in perspective made this speech one that still moves audiences all these years later.