It was a soothing color to see after so many stark shades of black and white. The president, who has long boasted that he doesn't see a lot of nuance, dressed up his speech on immigration in muted tones of gray.

Illegal immigrants, he said, live in "the shadows of our society," straining state and local budgets, pressuring schools and hospitals. They are also "decent people who work hard, support their families, practice their faith and lead responsible lives."

We got a brief glimpse of the man who had governed a state with a 1,200-mile border with Mexico and yet adamantly refused to ban the children of illegals from public schools. This time the president echoed the ambivalence among Americans who are evenly split on whether immigration helps or hurts the country. He talked about law enforcement and he talked about the melting pot. He spoke in tough words about using the National Guard to keep foreigners out. And he spoke gently about a wounded soldier's request for citizenship.

Listening, I was reminded of how long we have viewed immigrants as both heroes and threats. Every generation looks back through some sepia-tinged lens at the immigrants who built this country. It's the American story. But every generation also has those who look at new immigrants as threats to our national identity.

This was true when Benjamin Franklin rued the German immigrants who settled his native Pennsylvania. It was true when nativists insisted that the Irish and Italian and Jewish "races" were too foreign to ever assimilate. It's true now when many rise up in alarm at Latinos who sing the national anthem in Spanish.

Much of this debate is about chaos on the border. It's also about whether immigrants take jobs from citizens or do jobs Americans won't touch. But how much is about identity? About becoming American?

When my grandparents came to America, the door was open. Today, foreigners can only get permission to move here through family connections or job skills, as asylum-seekers or as winners in the annual visa lottery. We have what historian Roger Daniels calls a bipolar population of immigrants. And also of images. On the one hand, the foreign-born Ph.D. and will-be CEO. On the other hand, the person who cleans your house and mows your lawn, picks your lettuce and cares for your parents.

The melting pot is still as powerful an American icon as the Liberty Bell. "The success of our country," said Bush, "depends upon helping newcomers assimilate into our society and embrace our common identity as Americans." But here is where the president's shades of gray turn into the jarring colors of a policy patchwork.

The most troubling part of the proposals now on the table may not be using the National Guard — doomed from the get-go — nor the long, rocky road to citizenship some label amnesty. It may be the expanded "guest worker" program.

Many employers see this as a way to satisfy the need for temporary labor. Opponents see it as a way to undercut wages. What if it also undercuts the American story? Do we want to create a constitutionally sanctioned category of second-class citizens, or, rather un-citizens, or rather, never-to-be-citizens?

Historian David Hollinger of the University of California-Berkeley describes this as "a big deal" in our history, "dangerous and un-American." The traditional presumption was always that immigrants would come here and become full Americans. Some immigrants have always gone back and forth. Indeed, one in every three Europeans who came during the great waves of the 20th century — half of all the Italians — chose to go home.

But under this expanded program of tracked transients, we might well replace illegals who work "in the shadows" with guests who work in the permanent shade of discrimination.

In Europe, guest workers have long overstayed their welcome, if welcome is even the right word. They've become the disaffected source of trouble in countries that never much fancied the melting pot. It doesn't take much to imagine "guest workers" in America becoming exactly what illegal immigrants are now, a permanent subclass, and a living rebuke to the idea of assimilation.

"Americans are bound together by our shared ideals, an appreciation of our history, respect for the flag we fly, and an ability to speak and write the English language," said Bush. We are also a pluralistic society, and it's hard at times to define those "shared ideals." But an "appreciation of our history" will tell you that we were built and constantly renewed by newcomers who came to call the United States "my country" and defined themselves as citizens. Not as guests.

Ellen Goodman is a columnist for The Boston Globe. Reach her at ellengoodman@globe.com.